MULTI-FUNCTIONAL WINDOW ASSEMBLY HAVING WINDOWS EACH COMPOSED OF INDEPENDENT WINDOW PARTS Technical Field [1] The present invention relates, in general, to multi-functional window assemblies and, more particularly, to a multi-functional window assembly having windows each composed of independent window parts, wherein respective windows are composed of a combination of lattice window parts and vacant window parts alternated with each other, a combination of glass window parts and vacant window parts alternated with each other, or a combination of continuous vacant window parts, so that the entire window assembly can be maintained in an opened state under a specified window operating condition, with the lattice window parts creating various room environments, increasing security and performing an indirect ventilating function. Background Art [2] Generally, a window is provided so that a person or an object can pass or be moved through it. As well known to those skilled in the art, the conventional window assemblies are generally classified into a sliding window assembly, a casement window assembly, and a combined type window assembly. The sliding window assembly has a window frame and two or more windows which are arranged in a side- to-side relationship and opened and closed leftward and rightward. The casement window assembly has a frame and a single window which is opened and closed outward and inward about a hinge attached to an upper or side wall of the frame. The combined type window assembly employs both of the sliding window and the casement window. The window frame of each window is formed of wood, metallic material or PVC. Disclosure of Invention Technical Problem [3] In the case of the conventional sliding window assembly which is generally used in dwellings, a security issue is most significantly roused, and therefore, it is necessary to always pay attention to locking sliding windows. Also, since a security-increasing window is additionally positioned outside the sliding windows, an aesthetic appearance of the sliding window assembly is deteriorated. Further, while heating and cooling systems are universalized with the improvement of living standards, the con- ventional windows adopting a direct ventilation system cannot help but have a poor air-conditioning efficiency. Therefore, in this regard, when the heating and cooling systems are operated, it is demanded for the windows to be converted from the direct ventilation system to an indirect ventilation system so as to raise an air-conditioning efficiency. Technical Solution [4] Accordingly, the present invention has been made keeping in mind the above- mentioned problems occurring in the prior art, and an object of the present invention is to provide a multi-functional window assembly having windows each composed of in¬ dependent window parts, in which respective windows are composed of a combination of glass window parts and lattice window parts alternated with each other, a combination of glass window parts and vacant window parts alternated with each other, or a combination of continuous lattice window parts, so that the entire window assembly can be maintained in an opened state under a specified window operating condition, with the lattice window parts creating various room atmospheres and can perform a security increasing function and an indirect ventilating function. [5] Another object of the present invention is to provide a multi-functional window assembly having windows each composed of independent window parts, wherein respective windows are composed of a combination of lattice window parts and vacant window parts alternated with each other, a combination of glass window parts and vacant window parts alternated with each other, or a combination of continuous vacant window parts, so that the entire window assembly can be maintained in an opened state under a specified window operating condition, with the lattice window parts creating various room environments and performing a security-increasing function and an indirect ventilating function and so that, by unlocking an embedded-type locking device mounted to a main window or an auxiliary window and then sliding at least one necessary window, a person or an object can pass or be moved through the vacant window parts. [6] In an aspect, the present invention provides a multi-functional window assembly, including: an inner main window having first glass window parts and first lattice window parts which possess a predetermined width and are alternately joined with each other, and a first small-sized window part which possesses a width smaller than that of the first glass window parts and is positioned adjoining one end of the inner main window; an outer main window having a second small-sized window part, second lattice window parts and second glass window parts combined in that order as reversed to those of the inner main window, the second small-sized window part possessing a width smaller than that of the second glass window parts and being adjoining the other end of the outer main window, and the second lattice window parts and second glass window parts possessing a predetermined width and being alternately joined with each other; and a window frame in which the inner and outer main windows are slidably fitted and which is formed on its inner surface with rails and grooves for guiding the inner and outer main windows. [7] In another aspect, the present invention provides a multi-functional window assembly, including: a lattice window having lattice window parts and first vacant window parts which possess a predetermined width and are alternately joined with each other; an inner main window positioned outward of the lattice window and having first glass window parts and second vacant window parts which possess the same width as the lattice window parts and the first vacant window parts of the lattice window and are alternately joined with each other; and an outer main window positioned outward of the inner main window and having second glass window parts and third vacant window parts which possess the same width as the first glass window parts and the second vacant window parts of the inner main window. Advantageous Effects [8] In the multi-functional window assembly having windows each composed of in¬ dependent window parts according to the present invention, even when the window assembly is held opened by appropriately sliding inner and outer main windows and/or a lattice window, a crime-related issue is not roused. Also, when the window assembly is closed, it is unnecessary to lock the windows, by which the need for a separate crime-preventing window is obviated and thereby, an aesthetic appearance of the window assembly is not deteriorated. [9] Furthermore, in the multi-functional window assembly, by sliding the main windows, the lattice windows and the auxiliary window so that a degree to which the multi-functional window assembly is opened can be adjusted by small-sized window parts of the main windows, a complete integrated configuration capable of allowing the windows to be opened as desired is accomplished, and a security issue is not roused even when the window assembly is opened by sliding the windows to an end of the window frame. The lattice window parts create various room environments and perform a security-increasing function. Further, by unlocking an embedded type locking device mounted to the main window or the auxiliary window and then sliding at least one necessary window, a person or an object can pass or be moved through vacant window parts constituting the windows. Even in this case, an aesthetic appearance of the window assembly is not deteriorated and rather can be altered in a diversity of ways. [10] Further, in need of ventilation, by manipulating the auxiliary window, an indirect ventilation rate can be adjusted as desired. In the case that opaque window parts are provided in the main windows, it is possible to implement ventilation in a state where a privacy of a personal life is not impaired. Moreover, when the main windows are ma¬ nipulated to implement direct ventilation, since a plurality spaces are defined at several positions on the window assembly, a room can be quickly and properly ventilated in its entirety. Description of Drawings [11] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [12] FIG. 1 is a partially enlarged exploded perspective view illustrating a multi¬ functional window assembly having windows each composed of independent window parts, in accordance with a first embodiment of the present invention; [13] FIG. 2 is a front view illustrating component elements of the multi-functional window assembly shown in FIG. 1; [14] FIG. 3 is of perspective views illustrating operations of the multi-functional window assembly according to the first embodiment of the present invention; [15] FIG. 4 is a partially enlarged exploded perspective view illustrating a multi¬ functional window assembly having windows each composed of independent window parts, in accordance with a second embodiment of the present invention; [16] FIG. 5 is a front view illustrating component elements of the multi-functional window assembly shown in FIG. 4; [17] FIG. 6 is of perspective views illustrating operations of the multi-functional window assembly according to the first embodiment of the present invention; [18] FIG. 7 is of transverse cross-sectional views illustrating operations of the multi¬ functional window assembly according to the second embodiment of the present invention; [19] FIG. 8 is an exploded perspective view illustrating a multi-functional window assembly having windows each composed of independent window parts, in accordance with a third embodiment of the present invention; [20] FIG. 9 is a front view illustrating component elements of the multi-functional window assembly shown in FIG. 8; [21] FIG. 10 is of perspective views illustrating operations of the multi-functional window assembly according to the third embodiment of the present invention; [22] FIG. 11 is a partially enlarged exploded perspective view illustrating a multi¬ functional window assembly having windows each composed of independent window parts, in accordance with a fourth embodiment of the present invention; [23] FIG. 12 is a front view illustrating component elements of the multi-functional window assembly shown in FIG. 11; [24] FIG. 13 is a front view illustrating an assembling relationship among an auxiliary window and inner and outer lattice windows; [25] FIG. 14 is of front views illustrating operations of the multi-functional window assembly according to the fourth embodiment of the present invention; and [26] FIGS. 15 and 16 are of transverse cross-sectional views illustrating the operations of the multi-functional window assembly according to the fourth embodiment of the present invention. Best Mode [27] Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. [28] FIG. 1 is a partially enlarged exploded perspective view illustrating a multi¬ functional window assembly having windows each composed of independent window parts, in accordance with a first embodiment of the present invention. FIG. 2 is a front view illustrating component elements of the multi-functional window assembly shown in FIG. 1. The multi-functional window assembly according to this first embodiment of the present invention includes an inner main window a- 10, an outer main window a- 10', and a window frame a- 20. The inner main window a-10 has first glass window parts a-11 and first lattice window parts a- 12 which possess a predetermined width and are alternately joined with each other, and a first small-sized window part a- 13 which possesses a width smaller than that of the first glass window parts a-11 and is posit ioned adjoining one end of the inner main window a-10. The outer main window a-10' has a second small-sized window part a-13', second lattice window parts a-12' and second glass window parts a- 11' combined in that order as reversed to those of the inner main window a-10. The second small-sized window part a-13' possesses a width smaller than that of the second glass window parts a- 11' and is positioned adjoining the other end of the outer main window a-10'. The second lattice window parts a-12' and second glass window parts a- 11' possess a predetermined width and are alternately joined with each other. The inner and outer main windows a-10 and a-10' are slidably fitted in the window frame a- 20. The window frame a- 20 is formed on its inner surface with rails a-21 and grooves a- 22 for guiding the inner and outer main windows a-10 and a-10'. [29] Here, the width of the inner and outer main windows a-10 and a-10' is determined to be less by an amount of the width of the first and second small-sized window parts a- 13 and a- 13' than a width of the window frame a-20. [30] FIG. 3 is of perspective views illustrating operations of the multi-functional window assembly according to the first embodiment of the present invention. As can be readily seen from FIG. 3, when the first and second small-sized window parts a- 13 and a-13' of the inner and outer main windows a-10 and a-10' are positioned at both ends of the window frame a-20, the window assembly is closed. Further, when the glass window parts a-11 and a- 11' of the inner and outer main windows a-10 and a-10' are positioned at both ends of the window frame a-20, the window assembly is opened. [31] A window assembly according to a modification of the first embodiment will be described as follows, but not shown in the drawings. The window assembly according to the modification of the first embodiment includes an outer main window which has first glass window parts and first lattice window parts which possess a predetermined width and are alternately joined with each other. [32] The window assembly according to the modification further includes an inner main window having second glass window parts and second lattice window parts which are alternately joined with each other and possess the same width as the first glass window parts and the first lattice window parts of the outer main window. The inner main window has the number of the sum of the second glass window parts and the second lattice window parts fewer than the number of the sum of the first glass window and the first lattice window parts of the outer main window. [33] The window assembly according to the modification further includes a window frame in which the outer main window is fastened and the inner main window is slidably fitted, and which is provided on an inner surface thereof with both a rail and a groove for guiding a sliding movement of the inner main window. [34] The window assembly of the modification may further include an additional inner main window which has second glass window parts and second lattice window parts which are alternately joined with each other, possess the same width as the first glass window parts and the first lattice window parts of the outer main window, and has the number of the sum of the second glass window parts and the second lattice window parts fewer than the number of the sum of the first glass window parts and the first lattice window parts of the outer main window. The additional inner main window is slidably fitted in the window frame which is provided on the inner surface thereof with both an additional rail and an additional groove for guiding sliding movement of the additional inner main window. [35] FIG. 4 is a partially enlarged exploded perspective view illustrating a multi¬ functional window assembly having windows each composed of independent window parts, in accordance with a second embodiment of the present invention; and FIG. 5 is a front view illustrating component elements of the multi-functional window assembly shown in FIG. 4. A multi-functional window assembly according to this second embodiment of the present invention includes an inner main window a-30, a lattice window a-40, an outer main window a-30', and a window frame a-50. The inner main window a-30 has first glass window parts a-31 and first vacant window parts a-32 which possess a predetermined width and are alternately joined with each other, and a first small-sized window part a-33 which possesses a width smaller than that of the first glass window parts a-31 and is positioned adjoining one end of the inner main window a-30. The lattice window a-40 has lattice window parts a-41 which possess the same width as the first glass window parts a-31 and are joined one with another. The outer main window a-30' has a second small-sized window part a-33', second vacant window parts a-32' and second glass window parts a-31' combined in that order as reversed to those of the inner main window a-30. The second small-sized window part a-33' possesses a width smaller than that of the second glass window parts a-31' and is positioned adjoining the other end of the outer main window a-30'. The second vacant window parts a-32' and second glass window parts a-31' possess a predetermined width and are alternately joined with each other. The inner main window a-30, the lattice window a-40 and the outer main window a-30' are slidably fitted in the window frame a-50. The window frame a-50 is formed on its inner surface with rails a-51 and grooves a-52 for guiding the inner main window a-30, the lattice window a-40 and the outer main window a-30'. [36] The width of the inner and outer main windows a-30 and a-30' is determined to be less by an amount of the width of the first and second small-sized window parts a-33 and a-33' than a width of the window frame a-50. A width of the lattice window a-40 is determined to be less by an amount of the width of the first and second glass window parts a-31 and a-31' than the width of the window frame a-50. [37] Boundary regions between the glass window parts a-31 and a-31', the vacant window parts a-32 and a-32' and the small-sized window parts a-33 and a-33' of the inner and outer main windows a-30 and a-30' have the same thickness as the inner and outer main windows a-30 and a-30'. Boundary regions between the lattice window parts a-41 of the lattice window a-40 have the same thickness as the lattice window a- 40. [38] Inside the inner main window a-30, there may be provided a locking device for locking and unlocking the lattice window a-40, and signal receiving sensors may be embedded into the inside and outside main windows a-30 and a-30' and the lattice window a-40 so that sliding operations of the inside and outside main windows a-30 and a-30' and the lattice window a-40 can be controlled through a remote controller. [39] FIG. 6 is of perspective views illustrating operations of the multi-functional window assembly according to the first embodiment of the present invention. As can be readily seen from FIG. 6, when the first and second small-sized window parts a-33 and a-33' of the inner and outer main windows a-30 and a-30' are positioned at both ends of the window frame a-50, the window assembly is closed. Further, when the glass window parts a-31 and a-31' of the inner and outer main windows a-30 and a-30' are positioned at both ends of the window frame a-50, the window assembly is closed. [40] FIG. 7 is of transverse cross-sectional views illustrating operations of the multi¬ functional window assembly according to the second embodiment of the present invention. As described above, in the present embodiment of the present invention, the width of the inner and outer main windows a-30 and a-30' is determined to be less by an amount of the width of the first and second small-sized window parts a-33 and a- 33' than the width of the window frame a-50, a width of the lattice window a-40 is determined to be less by an amount of the width of the first and second glass window parts a-31 and a-31' than the width of the window frame a-50, the boundary regions between the glass window parts a-31 and a-31', the vacant window parts a-32 and a- 32' and the small-sized window parts a-33 and a-33' of the inner and outer main windows a-30 and a-30' have the same thickness as the inner and outer main windows a-30 and a-30', and the boundary regions between the lattice window parts a-41 of the lattice window a-40 have the same thickness as the lattice window a-40. Due to these facts, if the first and second small-sized window parts a-33 and a-33' of the inner and outer main windows a-30 and a-30' are positioned at both ends of the window frame a- 50 to close the window assembly, since the boundary regions between the lattice window parts a-41 have the same thickness as the lattice window a-40, the lattice window a-40 prevents air from passing through the window assembly to render a heat insulation effect (Sl). In the first step Sl, by sliding the lattice window a-40, all windows are converted from the closed state to an indirect ventilation mode (S2). In the second step S2, if one of the first and second glass window parts a-31 and a-31' of the inner and outer main windows a-30 and a-30' is positioned at one end of the window frame a-50, the window assembly is opened by an amount of the width of the first and second small-sized window parts a-33 and a-33' (S3, S4 and S5). In the fifth step S5, if the glass window parts a-31 and a-31' of the inner and outer main windows a-30 and a-30' are positioned at both ends of the window frame a-50, the window assembly is opened, and at this time, the lattice window a-40 increases security. [41] FIG. 8 is an exploded perspective view illustrating a multi-functional window assembly having windows each composed of independent window parts, in accordance with a third embodiment of the present invention; and FIG. 9 is a front view il¬ lustrating component elements of the multi-functional window assembly shown in FIG. 8. A multi-functional window assembly according to this third embodiment of the present invention includes a lattice window b-10, an inner main window b-20 and an outer main window b-30. The lattice window b-10 has lattice window parts b-11 and first vacant window parts b-12 which possess a predetermined width and are al¬ ternately joined with each other. The inner main window b-20 is positioned outward of the lattice window b-10. The inner main window b-20 has first glass window parts b- 21 and second vacant window parts b-22 which possess the same width as the lattice window parts b-11 and the first vacant window parts b-12 of the lattice window b-10 and are alternately joined with each other. The outer main window b-30 is positioned outward of the inner main window b-20. The outer main window b-30 has second glass window parts b-31 and third vacant window parts b-32 which possess the same width as the first glass window parts b-21 and the second vacant window parts b-22 of the inner main window b-20. [42] Here, a width of each of the lattice window b-10 and inner main window b-20 is determined to be less than that of the outer main window b-30 by the sum of respective widths of the second glass window part b-31 and third vacant window part b-32. [43] FIG. 10 is of perspective views illustrating operations of the multi-functional window assembly according to the third embodiment of the present invention. As can be readily seen from FIG. 10, when the leftmost first glass window part b-21 of the inner main window b-20 is overlapped with the leftmost third vacant window part b-32 of the outer main window b-30 and the leftmost lattice window part b-11 of the lattice window b-10 is overlapped with the leftmost second vacant window part b-22 of the inner main window b-20, the entire window assembly is closed. In this closed state, if the inner main window b-20 is slidingly moved such that a left end of the inner main window b-20 is positioned to a left end of the outer main window b-30, the window assembly is opened in a security-increasing state. In this security-increasing opened state, by unlocking a locking device embedded into the inner main window b-20 and then positioning a left end of the lattice window b-10 to the left end of the outer main window b-30, the window assembly is opened so that a person or an object can pass or be moved through the vacant window parts b-12, b-22 and b-32. [44] FIG. 11 is a partially enlarged exploded perspective view illustrating a multi¬ functional window assembly having windows each composed of independent window parts, in accordance with a fourth embodiment of the present invention; and FIG. 12 is a front view illustrating component elements of the multi-functional window assembly shown in FIG. 11. A multi-functional window assembly according to this fourth embodiment of the present invention includes an inner lattice window b-40, an inner main window b-50, an auxiliary window b-63, an outer main window b-50', an outer lattice window b-40' and a window frame b-70. The inner lattice window b-40 has first lattice window parts b-41 and first vacant window parts b-42 which possess a pre¬ determined width and are alternately joined with each other. The inner main window b-50 is positioned outward of the inner lattice window b-40. The inner main window b-50 has first glass window parts b-51 and second vacant window parts b-52 which possess the same width as the first lattice window parts b-41 and the first vacant window parts b-42 of the inner lattice window b-40 and are alternately joined with each other, and a first small-sized window part b-53 which possesses a width smaller than that of the first glass window part b-51 and is positioned at one end of the inner main window b-50. The auxiliary window b-60 has third vacant window parts b-61 which possess the same width as the first glass window parts b-51 of the inner main window b-50 and are continuously joined one with another. The outer main window b- 50' has a second small-sized window part b-53', fourth vacant window parts b-52' and second glass window parts b-51' which are combined in that order as reversed to those of the inner main window b-50. The fourth vacant window parts b-52' and second glass window parts b-51' possess the predetermined width and are alternately joined with each other. The second small-sized window part b-53' possesses a width smaller than that of the second glass window part b-51 ' and is positioned at the other end of the outer main window b-50'. The outer lattice window b-40' is positioned outward of the outer main window b-50'. The outer lattice window b-40' has second lattice window parts b-41' and fifth vacant window parts b-42' which possess the same width as the second glass window parts b-51' and fourth vacant window parts b-52' of the outer main window b-50' and are alternately joined with each other. The inner main window b-50, the auxiliary window b-60 and the outer main window b-50' are slidably fitted in the window frame b-70. The window frame b-70 is formed on its inner surface with rails 71 and grooves 82 for guiding sliding movement of the inner main window b-50, the auxiliary window b-60 and the outer main window b-50'. [45] Here, a width of each of the inner and outer lattice windows b-40 and b-40' is determined to be less than that of the window frame b-70 by the sum of respective widths of the first or second lattice window part b-41 or b-41' and first or fifth vacant window part b-42 or b-42'. A width of each of the inner and outer main windows b-50 and b-50' is determined to be less than that of the window frame b-70 by a width of the first or second small-sized window part b-53 or b-53'. A width of the auxiliary window b-60 is determined to be less than that of the window frame b-70 by a width of the third vacant window part b-61. [46] Further, boundary portions between the lattice window parts b-41 and b-41' and the vacant window parts b-42 and b-42' which constitute the inner and outer lattice windows b-40 and b-40' have the same thickness as the inner and outer lattice windows b-40 and b-40'. Boundary portions between the glass window parts b-51 and b-51', the vacant window parts b-52 and b-52' and the small-sized window parts b-53 and b-53' which constitute the inner and outer main windows b-50 and b-50' have the same thickness as the inner and outer main windows b-50 and b-50'. Boundary portions between the vacant window parts b-61 which constitute the auxiliary window b-60 have the same thickness as the auxiliary window b-60. [47] Inside the main window b-50 or b-50', there may be provided a locking device for locking and unlocking the lattice window b-40 or b-40', and signal receiving sensors may be embedded into the inner and outer lattice windows b-40 and b-40', the inner and outer main windows b-50 and b-50', the auxiliary window b-60 so that sliding operations of the inner and outer lattice windows b-40 and b-40', the inner and outer main windows b-50 and b-50' and the auxiliary window b-60 can be controlled through a remote controller. [48] FIG. 14 is of front views illustrating operations of the multi-functional window assembly according to the fourth embodiment of the present invention. In a state wherein the first and second lattice window parts b-41 and b-41' of the inner and outer lattice windows b-40 and b-40' are overlapped with the second and fourth vacant window parts b-52 and b-52' of the inner and outer main windows b-50 and b-50', re¬ spectively, if the first and second small-sized window parts b-53 and b-53' of the inner and outer main windows b-50 and b-50' are positioned at both ends of the window frame b-70, respectively, the entire window assembly is maintained in a closed state in which all the windows are completely closed. In this completely closed state, if the inner and outer main windows b-50 and b-50' are slidingly moved such that the first and second window parts b-51 and b-51' provided at respective opposing ends of the inner and outer main windows b-50 and b-50' are positioned at both ends of the window frame b-70, respectively, the window assembly is opened in a security- increasing state. In this security-increasing opened state, by unlocking a locking device embedded into the inner main window b-50 and then sliding the inner and outer lattice windows b-40 and b-40' so that the first and second lattice window parts b-41 and b- 41' provided at respective opposing ends of the inner and outer lattice windows b-40 and b-40' are overlapped with the first and second window parts b-51 and b-51' provided at respective opposing ends of the inner and outer main windows b-50 and b- 50', respectively, the window assembly is opened so that a person or an object can pass or be moved through the vacant window parts b-42, b-52, b-61, b-52' and b-42'. [49] FIG. 13 is a front view illustrating an assembling relationship among the auxiliary window and the inner and outer lattice windows. The inner and outer lattice windows b-40 and b-40' are structured so that they can be selectively fitted in the inner and outer main windows b-50 and b-50' or the auxiliary window b-60. In the latter case, the first and second lattice window parts b-41 and b-41' provided at the respective opposing ends of the inner and outer lattice windows b-40 and b-40' can be overlapped with the third vacant window parts b-61 which are provided at both ends of the auxiliary window b-60, respectively. [50] In the window assembly constructed as mentioned above, in a state wherein the inner and outer lattice windows b-40 and b-40' are positioned at both ends, re¬ spectively, of the auxiliary windows b-60, if the first and second small-sized window parts b-53 and b-53' of the inner and outer main windows b-50 and b-50' are re¬ spectively slid to both ends of the window frame b-70 and the auxiliary window b-60 is slid so that both ends of the auxiliary window b-60 are overlapped with the first and second window parts b-51 and b-51' which are provided at the respective opposing ends of the inner and outer main windows b-50 and b-50', the entire window assembly is closed. In this closed state, if the first and second window parts b-51 and b-51' provided at the respective opposing ends of the inner and outer main windows b-50 and b-50' are positioned at both ends of the window frame b-70, respectively, the window assembly is opened in the security-increasing state. In this security-increasing opened state, by unlocking a locking device embedded into the auxiliary window b-60 and then sliding the inner lattice window b-40 so that it can be overlapped with the outer lattice window b-40', the window assembly is opened so that a person or an object can pass or be moved through the vacant window parts b-42, b-52, b-61, b-52' and b-42'. [51] FIGS. 15 and 16 are of transverse cross-sectional views illustrating the operations of the multi-functional window assembly according to the fourth embodiment of the present invention. As described above, in the present embodiment of the present invention, the width of each of the inner and outer lattice windows b-40 and b-40' is determined to be less than that of the window frame b-70 by the sum of respective widths of the first or second lattice window part b-41 or b-41' and first or fifth vacant window part b-42 or b-42', the width of each of the inner and outer main windows b- 50 and b-50' is determined to be less than that of the window frame b-70 by a width of the first or second small-sized window part b-53 or b-53', and the width of the auxiliary window b-60 is determined to be less than that of the window frame b-70 by a width of the third vacant window part b-61. Further, the boundary portions between the lattice window parts b-41 and b-41' and the vacant window parts b-42 and b-42' which constitute the inner and outer lattice windows b-40 and b-40' have the same thickness as the inner and outer lattice windows b-40 and b-40', boundary portions between the glass window parts b-51 and b-51', the vacant window parts b-52 and b- 52' and the small-sized window parts b-53 and b-53' which constitute the inner and outer main windows b-50 and b-50' have the same thickness as the inner and outer main windows b-50 and b-50', and boundary portions between the vacant window parts b-61 which constitute the auxiliary window b-60 have the same thickness as the auxiliary window b-60. Due to these facts, in a state wherein the first and second lattice window parts b-41 and b-41' provided at the respective opposing ends of the inner and outer lattice windows b-40 and b-40' are respectively overlapped with the second and fourth vacant window parts b-52 and b-52' of the inner and outer main windows b-50 and b-50', if the first and second small-sized window parts b-53 and b- 53' of the inner and outer main windows b-50 and b-50' are respectively positioned at both ends of the window frame b-70, all the windows are maintained in the completely closed state (Sl). In the first step Sl, by sliding the auxiliary window b-60, indirect ventilation is implemented with all the windows substantially closed (S2). In the second step S2, by sliding the first or second window parts b-51 or b-51' provided at the respective opposing ends of the inner and outer main windows b-50 and b-50' so that each of them is positioned at an end of the window frame b-70, the window assembly is opened through a space defined by the small-sized window parts b-53 or b-53' (S3 or S4). In the third or fourth step S3 or S4, by sliding the first or second window parts b-51 or b-51' provided at the respective opposing ends of the inner and outer main windows b-50 and b-50 so that each of them is positioned at an end of the window frame b-70, the window assembly is opened in the security-increasing state (S5 or S6). In the fifth or sixth step S5 or S6, by unlocking the locking device embedded into the inner main window b-50 and then sliding the inner or outer lattice windows b-40 or b-40' so that the first or second lattice window parts b-41 or b-41' provided at the respective opposing ends of the inner and outer lattice windows b-40 and b-40' is overlapped with the first or second window parts b-51 or b-51' provided at the respective opposing ends of the inner and outer main windows b-50 and b-50', respectively, the window assembly is opened so that a person or an object can pass or be moved through the vacant window parts b-42, b-52, b-61, b-52' and/or b-42'(S7 or S8). Mode for Invention [52] Industrial Applicability [53] As apparent from the above description, the multi-functional window assembly having windows each composed of independent window parts according to the present invention provides advantages in that, even when the window assembly is in an open position by appropriately sliding inner and outer main windows and/or a lattice window, a security-related issue is not roused. Also, when the window assembly is closed, it is unnecessary to lock the windows, by which the need for a separate security increasing window is obviated and thereby, an aesthetic appearance of the window assembly is not deteriorated. [54] Furthermore, in the multi-functional window assembly, by sliding the main windows, the lattice windows and an auxiliary window, a degree to which the multi¬ functional window assembly is opened can be adjusted by small-sized window parts of the main windows. Consequently, a complete integrated configuration capable of allowing the windows to be opened as desired is accomplished, and a security issue is not roused even when the window assembly is opened by sliding the windows to an end of a window frame. Also, lattice window parts of the lattice windows create various room environments and perform a security-increasing function. Further, by unlocking an embedded type locking device mounted to the main window or the auxiliary window and then sliding at least one necessary window, a person or an object can pass or be moved through vacant window parts of the windows. Even in this case, the aesthetic appearance of the window assembly is not deteriorated and rather can be altered in a diversity of ways. [55] Further, in need of ventilation, by manipulating the windows, an indirect ventilation amount can be adjusted as desired. In the case that opaque window parts are provided in the windows, it is possible to implement ventilation in a state where privacy is not impaired. [56] Moreover, when the main windows are manipulated to implement direct ventilation, since a plurality spaces are defined at several positions on the window assembly, a room can be quickly ventilated in its entirety. [57] Furthermore, a monolithic configuration of the conventional window assemblies can be changed into a diversity of configurations. In addition, since a variety of lattice patterns can be used, an aesthetic appearance of the window assembly can be ameliorated. Moreover, when a digital technique is applied to the window assembly of the present invention, an artificial intelligence-type control of the window assembly is realized, thus automatically controlling the windows. [58] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modi¬ fications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.