APPARATUS OF PRODUCING BREATHABLE FILM Technical Field [1] The present invention generally relates to an apparatus of producing a breathable film, and more particularly, to an apparatus for producing a breathable film that allows gas such as vapor to permeate therethrough but inhibits liquid penetration. Background Art [2] Breathable films are a kind of plastic product having a myriad of fine pores and have a property of inhibiting penetration of liquid while allowing gas, including vapor, to pass therethrough. Such breathable films are widely used as diapers, sanitary napkins, disposable working dresses, fruit sacks, materials for building and the like. [3] The breathable films can be produced by virtue of a film production apparatus. Conventional film production apparatus is composed of a hopper through which pelletized resin mixture materials can be supplied, an extruder for melting and extruding the resin mixture materials in the form of a sheet or tube, a shaper for forming the extruded resin of sheet or tube shape into a film, an elongating machine for extending the length of the shaped film, a surface treatment machine for treating the surface of the elongated film and a take-up device for taking up the surface-treated film in the form of a roll. Particularly, the shaper is provided with a shaping roller that serves to guide and cool down the extruded molten resin as it is extruded in the form of a sheet or tube. [4] With this type of film production apparatus, a variety of resin materials are mixed at a proper ratio and then pelletized. The pelletized resin materials are put into the hopper, after which the resin materials pass through the extruder and are extruded in the form of a sheet or tube. The extruded molten resin materials are shaped into a sheet-like or tube-like film by the shaper. During this process of melting and extruding the resin materials, such additives as an antioxidant, a permeability-endowing agent and a dehumidifying agent are added if desired. [5] The shaped film is elongated into a predetermined length by the elongating machine and then surface-treated by the surface treatment machine, at the end of which the film is taken up into a roll of breathable film. [6] The prior art film production apparatus requires that various kinds of resin materials have to be mixed at a proper ratio and pelletized before they are put into the apparatus. Therefore, a need exists for a separate working process of preliminarily formulating and mixing the resin materials. [7] This makes the film production process bifurcated into a step of formulating and mixing the resin materials and a step of shaping a film, which leads to a decreased continuity of process and a reduced working efficiency and manufacturability. In addition to the above, an independent device has to be used in order to formulate and mix the resin materials, thus making the film production apparatus costly. [8] Moreover, a moisture removing process is required in the prior art film production apparatus, due to the fact that a large quantity of moisture tends to be absorbed in the process of pelletizing the resin materials. [9] Additionally, the prior art film production apparatus has a tendency to change the property of the film while the film is shaped by means of the shaping roller. More specifically, the shaping roller is adapted to guide and cool down the resin materials of elevated temperature into the film as the resin materials are extruded from the extruder. In this process, the shaping roller makes contact with and speedily cools down one surface of the film. This makes the cooling speed of one surface of the film different from that of the other surface, thus changing the film property from surface to surface. [10] Furthermore, the prior art film production apparatus is designed to elongate the film under the condition that the film is not completely dried. This reduces film elongation efficiency and results in an elongated film of uneven thickness, which may be a cause of producing a film of poor quality. Disclosure of Invention Technical Problem [11] Taking the afore-mentioned problems inherent in the prior art film production apparatus into account, it is an object of the present invention to provide a breathable film production apparatus that can combine a resin material mixing process and a film forming process into a single process, thus assuring continuity of overall process and improved film manufacturability. [12] Another object of the present invention is to provide a breathable film production apparatus that can produce a film of excellent quality without resort to the separate process of removing moisture. [13] A further object of the present invention is to provide a breathable film production apparatus that can guide and cool down opposite surfaces of the film discharged from an extruder, thereby making the property of the film uniform. [14] A still further object of the present invention is to provide a breathable film production apparatus designed to elongate a completely dried film, thus making it possible to increase the elongation efficiency and produce a film of uniform thickness. [15] In order to achieve the above objects, the present invention provides an apparatus of producing a breathable film, comprising: a mixer for mixing resin materials with an an¬ tioxidant and a permeability-endowing agent; an extruder for melting and extruding the resin materials mixed in the mixer; a shaper for forming the resin materials extruded from the extruder into a film; a drier for causing the film formed in the shaper to run dry; an elongating machine for extending the length of the film dried in the drier; a surface treatment machine for treating surfaces of the film with discharge of electricity; and a take-up device for taking up the film treated in the surface treatment machine into a roll of film. [16] It is preferred that the mixer comprises: a plurality of hoppers through which the resin materials and the antioxidant are supplied into the apparatus; a casing having an internal passageway for receiving the resin materials and the antioxidant from the hoppers; a screw rotatably mounted in the passageway of the casing for mixing and discharging the resin materials and the antioxidant received in the passageway; a heater attached to the casing for melting the resin materials as they are moved along the passageway; and a permeability-endowing agent supply hopper attached to the casing for feeding the permeability-endowing agent to the molten resin materials that move along the passageway. [17] It is also preferred that the mixer has a vent hole formed through the casing for removing gas and moisture generated from the molten resin materials and a compulsory ventilation means attached to the vent hole for forcibly exhausting the gas and moisture to the outside. Brief Description of the Drawings [18] Fig. 1 is a view showing an apparatus of producing a breathable film in accordance with the present invention. [19] Fig. 2 is a view illustrating a mixer, an extruder and a shaper incorporated in the breathable film production apparatus of the present invention, wherein an extruding die of the type for producing a sheet-like film is employed. [20] Fig. 3 is a view showing a modified example of the breathable film production apparatus of the present invention, wherein an extruding die of the type for producing a tube-like film is employed. [21] Fig. 4 is a view illustrating a drier, an elongating machine and a take-up machine in¬ corporated in the breathable film production apparatus of the present invention. Best Mode for Carrying Out the Invention [22] A preferred embodiment of the breathable film production apparatus according to the present invention will now be described in detail with reference to the ac¬ companying drawings. [23] Referring first to Fig. 1, it can be seen that the breathable film production apparatus according to the present invention comprises a mixer 10 for mixing pelletized resin materials, an extruder 20 for melting and extruding the resin materials mixed, a shaper 30 for forming the extruded resin materials into a film, a drier 40 for causing the shaped film F to run dry; an elongating machine 50 for extending the length of the dried film F, a surface treatment machine 60 for treating surfaces of the elongated film F, and a take-up device 80 for taking up the surface-treated film into a roll of film. [24] More specifically, as shown in Fig. 2, the mixer 10 is provided with a casing 12 of cylindrical configuration which in turn includes a passageway 13 having an inlet 13a at one side and an outlet 13b at the other side. [25] At the inlet 13a of the casing 12, there is provided a plurality of hoppers 14a, 14b, 14c through which pelletized resin materials can be supplied. The hoppers 14a, 14b, 14c are designed to, at their lower ends, converge toward a connector pipe 14d which is mounted to the inlet 13a of the casing 12. [26] Each of the hoppers 14a, 14b, 14c, the lower ends of which converge at the inlet 13a of the casing 12, serves to introduce various kinds of resin materials into the passageway 13 of the casing 12. It is preferred that the hoppers 14a, 14b, 14c are three in number, each referred to as a first hopper 14a, a second hopper 14b and a third hopper 14c hereinbelow. The first hopper 14a is adapted to supply, for example, low density polyethylene as a raw material for the breathable film, the second hopper 14b supplying polypropylene, high density polyethylene and linear low density polyethylene as another raw material for the breathable film, and the third hopper 14c supplying antioxidant as an additive. [27] The mixer 10 is provided with a screw 15 extending within and along the passageway 15. The screw 15 can be rotatingly driven by a drive motor 15a attached to one side of the casing 12 so that it mix the resin materials supplied from the inlet 13a and transport the mixed resin materials toward the outlet 13b. [28] Furthermore, the mixer 10 is provided with a heater 16 mounted around the casing 12. The heater 16 serves to heat the casing 12 to thereby melt the resin materials that move along the passageway 13 of the casing 12. The resin materials are heated up at a temperature of about 180-250°C by the heater 16. The heating temperature is so selected to become about 180°C at the upstream of the passageway 13, about 200-230°C at the midway of the passageway 13 and about 250°C at the downstream of the passageway 13. This makes sure that the resin materials introduced into the passageway 13 are heated stepwise with an increased melting efficiency. [29] Additionally, the mixer 10 has a permeability-endowing agent inlet port 17 through which a permeability-endowing agent, e.g., calcium carbonate (CaCO ), can be introduced into the passageway 13 of the casing 12. A hopper 17a is mounted to the inlet port 17. It is preferred that the permeability-endowing agent inlet port 17 is positioned at the middle part of the passageway 13. This assures that the permeability- endowing agent can be added to the resin materials when they are in the completely molten condition, thereby enhancing the humidity removing efficiency. In the following description, the molten resin materials will be referred to simply as "molten resin". [30] Moreover, the mixer 10 is provided with a vent hole 18 through which gas and moisture generated from the molten resin can be removed. It is preferred that the vent hole 18 is located at the downstream side of the passageway 13. This is to facilitate removal of the gas and moisture from the molten resin. [31] Provided at the vent hole 18 is a compulsory ventilation means 18a that serves to forcibly exhaust the gas and moisture emitted from the molten resin. Examples of the compulsory ventilation means 18a include a vacuum pump that can forcibly suck up and exhaust the gas and moisture emitted from the molten resin with an increased efficiency. [32] The mixer 10 is further provided with a discharge pipe 19 through which the molten resin is discharged from the outlet 13b. The discharge tube 19 is connected to the extruder 20 set forth later and serves to transfer the molten resin from the outlet 13b to the extruder 20. [33] The mixer 10 as constructed above plays a role of mixing and discharging various kinds of resin materials, causing the mixed resin materials to be melted at a proper temperature, allowing an additive to be added to the molten resin materials whereby the property of the molten resin can be adjusted to a better condition for producing a breathable film. In addition, it will be possible for the mixer 10 to speedily remove the gas and moisture emitted from the molten resin, thus increasing the purity of the molten resin. [34] Referring again to Figs. 1 and 2, the inventive film production apparatus comprises an extruder 20 connected to the mixer 10. The extruder 20 is provided with a casing 22 of cylindrical configuration that includes an internal passageway 23 having an inlet 23a at one side and an outlet 23b at the other end. [35] Disposed at the inlet 23a of the casing 22 is an introduction pipe 24 which in turn is connected to the discharge pipe 19 of the mixer 10. The introduction pipe 24 connected to the discharge pipe 19 is of such a construction that it can introduce the molten resin discharged from the mixer 10 into the passageway 23 of the casing 22. [36] The introduction pipe 24 has an additive inlet port 25 to which a hopper 25a is mounted for accommodating additives. The additive inlet port 25 and the hopper 25a are used to introduce such additives as an antioxidant and a permeability-endowing agent into the passageway 23 of the casing 22. [37] The extruder 20 is provided with a screw 26 extending through the passageway 23. The screw 26 is caused to rotate by a drive motor 26a attached to one side of the casing 22 and serves to transport the molten resin from the inlet 23a to the outlet 23b. [38] Furthermore, the extruder 20 is provided with a heater 27 mounted around the casing 22. The heater 27 serves to heat the molten resin that moves along the passageway 23, to a temperature of about 190-230°C. The heating temperature is so selected to become about 190°C at the upstream of the passageway 23, about 200°C at the midway of the passageway 23 and about 230°C at the downstream of the passageway 23. [39] The extruder 20 is further provided with an extruding die 28 attached to the outlet 23b of the passageway 23. The extruding die 28 serves to extrude the molten resin discharged from the outlet 23b in the form of a sheet. [40] As an alternative for the extruding die 28 illustrated in Figs. 1 and 2, the extruding die may be modified to the one as shown in Fig. 3 that can extrude the molten resin in the form of a tube. The extruding die 28 of this modified example is adapted to extrude the molten resin in a circular form. Provided around the extruding die 28 are a blower 28a and a nozzle 28b that blow air into the extruded resin to inflate it in the form of a tube. [41] With the extruder 20 as constructed above, it becomes possible to heat the molten resin supplied from the mixer 10 and extrude it in the form of a sheet or tube. [42] Referring collectively to Figs. 1 through 3, the film production apparatus according to the invention comprises a shaper 30 for forming the resin extruded from the extruder 20. [43] The shaper is composed of a first shaping roller 32 for contacting and shaping one surface of the resin extruded from extruding die 28 and a second shaping roller 34 for contacting and shaping the other surface of the resin transferred from the first shaping roller 32. Particularly, each of the first and second shaping rollers 32, 34 is adapted to shape the resin while cooling down the corresponding one surface of the extruded resin. [44] Such a shaper 30 serves to guide and cool down the sheet-like or tube-shaped resin extruded from the extruder 20 to thereby shape the resin into a sheet-like or tube- shaped film F. Particularly, the shaper 30 can maintain the property of the shaped film F uniform by way of successively and alternately contacting and cooling down the opposite surfaces of the extruded resin. [45] Referring to Figs. 1 and 4, the film production apparatus according to the invention comprises a drier 40 for causing the shaped film F to run dry. The drier 40 is provided with a plurality of guide rollers 42 for guiding the shaped film F in zigzags and a blower 44 for injecting dry air heated up to a temperature of 60-100°C into the film F guided in zigzags. [46] Such a drier 40 can speedily dry the film F by way of blowing air to the film F as it is guided in zigzags. Particularly, the air blowing helps increase the efficiency of drying the film F, thus assuring that the elongation efficiency is improved during the process of elongating the film F. [47] Referring again to Figs. 1 and 4, the film production apparatus according to the invention comprises an elongating machine 50 for extending the length of the dried film F. [48] The elongating machine 50 is provided with a plurality of elongating rollers 52, a heater 54 for preheating the film F into a suitable condition for elongation as the film F is moved along the elongating rollers 52, and a cooler 56 for cooling down the elongated film F. [49] This elongating machine 50 reduces the elongation rate but increases the strength of the film F by extending the length of the film F that passes therethrough. The elongation efficiency of the film F can be increased because the film F is elongated by the elongating machine 50 under a sufficiently dried condition. Moreover, the increased elongation efficiency provides a film of uniform thickness. [50] Referring back to Figs. 1 and 4, the film production apparatus according to the invention comprises a surface treatment machine 60 for treating the surfaces of the elongated film F with electrical discharge and an inspection device 70 for allowing the operator to inspect the surface-treated film F. [51] The surface treatment machine 60 includes a corona discharger that can treat the surfaces of the film F by discharge of electricity. By treating the surfaces of the film F with electrical discharge, an adhesive agent can be readily coated on the film surfaces and the coated adhesive agent is prohibited from releasing out of the film F. The corona discharger is well known in the art and therefore no description will be offered in this regard. [52] The inspection device 70 is provided with a plurality of guide rollers 72 that serves to guide the film F along a predetermined travel section in a spread condition. The operator can inspect defects of the film F with naked eyes as it is moved in the spread condition. [53] Referring again to Figs. 1 and 4, the film production apparatus according to the invention comprises a take-up device 80 adapted to take up the film F whose inspection has been completed. The take-up device 80 a plurality of guide rollers 82 that serves to guide the film F transferred from the inspection device 70 and a take-up roller 84 that functions to wind the film F into a roll. The take-up device 80 plays a role of winding the inspected film F for easier transportation and storage. [54] In the following, description will be offered as to the operation of the film production apparatus as constructed above with reference to Figs. 1 and 3. [55] The apparatus is first turned on through the use of a switch not shown in the drawings. Subsequently, resin materials are put into the first hopper 14a and the second hopper 14b while an antioxidant is introduced into the third hopper 14c. The resin materials and the antioxidant contained in the first through third hoppers 14a, 14b, 14c are supplied to the internal passageway 13 of the mixer 10 in which they are mixed with each other and caused to move by the screw 15. [56] The resin materials moving along the passageway 13 are gradually melted by the heater 16 attached to the casing 12 and the molten resin materials moves toward the outlet 13b at a reduced speed. As the resin materials begin to be melted, calcium carbonate as a permeability-endowing agent is fed through the permeability-endowing agent inlet port 17. Concurrently with the introduction of the permeability-endowing agent, the gas and the moisture emitted from the molten resin materials are exhausted to the outside through the vent hole 18 by means of the compulsory ventilation means 18a. [57] At the outlet 13b, the molten resin is transferred to the extruder 20 via the discharge pipe 19 and the introduction pipe 24, after which the molten resin in the extruder 20 is caused to move toward the outlet 23a by means of the screw 26. If needed, such additives as the antioxidant and the permeability-endowing agent are additionally supplied through inlet port 25 of the introduction pipe 24. [58] When moved to the outlet 23b, the molten resin passes through the extruding die 28 and is extruded into a form of sheet or tube. The extruded resin is brought into contact with the first and second shaping rollers 32, 34 one after the other and shaped into a film F. At this moment, the opposite surfaces of the film F are uniformly cooled down in the course of passing through the first and second shaping rollers 32, 34, thereby assuring that the film F exhibits uniform property. [59] The film F that has passed the shaper 30 is transferred to the drier 40 in which the film F moves in zigzags and runs dry. In order to speedily dry the film F, air is supplied to the surfaces of the film F by the blower 44 during the zigzag movement of the film F. [60] The dried film F is elongated to a predetermined length by means of the elongating machine 50 and then surface-treated with the surface treatment machine 60, after which the elongated and surface-treated film is inspected with the naked eyes of the operator while passing through the inspection device 70. At the end of inspection, the film F is finally wound into a roll by the take-up roller 84 of the take-up device 80, thus providing a breathable film. Industrial Applicability [61] As described above, the breathable film production device according to the present invention is provided with a built-in mixer, which makes it possible to combine a resin material mixing process and a film forming process into a single process, thus assuring continuity of overall process and improved film manufacturability. The opposite surfaces of the film discharged from an extruder can be cooled down evenly, thereby making the property of the film uniform. A completely dried film is subjected to elongation, thus making it possible to increase the elongation efficiency and produce a film of uniform thickness. In addition to the above, the mixing and forming processes are conducted in a single apparatus, meaning that there is no need to mix and pelletize the resin materials. Omission of the pelletizing process prevents the resin material from absorbing moisture.