FIELD OF THE INVENTION The present invention relates to a thread feeding apparatus for an automatic embroidering machine, and more particularly, to a thread feeding apparatus for an automatic embroidering machine having an improved structure of feeding a thread take-up unit with an upper thread. BACKGROUND ART An automatic embroidering machine is a kind of a sewing machine, which uses various colored threads and automatically embroiders a predetermined pattern, a trademark, or the like on clothes, a label, shoes, etc. according to a preset program. In general, the automatic embroidering machine includes a thread supplying unit to supply various colored upper threads from a plurality of bobbins toward an embroidering work place; a plurality of needle-working units to receive the upper threads from the thread supplying unit and perform embroidering works by one needle-working unit selected among them; and a plurality of thread take-up units to strengthen and release the tension of the upper thread connected to each needle- working unit. Here, each needle-working unit includes a needle reciprocating up and down to perform the embroidering works, and a presser foot to press a fabric along a working direction of the needle. Such automatic embroidering machine includes the plurality of needle- working units, the plurality of thread take-up units, -etc. so that its structure is complicated. To improve the complicated structure of the automatic embroidering machine, there has been developed an automatic embroidering machine for a single needle, which includes a single needle worktable. For example, a thread feeding apparatus for a single needle automatic embroidering machine has been disclosed in Korean Utility Model No. 20-167993. The conventional thread feeding apparatus for the single needle automatic embroidering machine includes a vertical operating pipe provided to receive one selected among the plurality of upper threads from a thread supplying unit; a holding pipe to receive the upper thread from the vertical operating pipe; and an air- supplying nozzle provided in the end of the holding pipe and injecting high-pressure air to transfer the upper thread toward the needle-working unit. Therefore, the upper threads are automatically fed according to colors . Meanwhile, in the case that the upper thread is replaced as a color is changed while embroidering or the upper thread is cut, a thread retainer of the thread take-up unit is rotated and disposed behind an axis formed from a bottom dead point of the thread take-up unit to the vertical operating pipe and the holding pipe, i.e., behind a thread transferring path. Then, the vertical operating pipe and the holding pipe are coupled to form a connection pipe path, and then the upper thread is supplied to the connection pipe path. Thereafter, the vertical operating pipe and the holding pipe are spaced apart from each other at a predetermined distance, and then the thread retainer of the thread take-up unit is rotated forward the thread transferring path through a gap between the vertical operating pipe and the holding pipe, so that the thread is caught on the thread retainer of the thread take-up unit. Thus, to replace the upper thread in such conventional thread feeding apparatus, the thread retainer of the thread take-up unit is rotated beyond an operating range between a top dead point and the bottom dead point of the thread take-up unit and disposed behind the thread transferring path, and then the thread is taken-up by the thread retainer while the thread retainer passes through the thread transferring path by moving from the back of the thread transferring path toward the bottom dead point. Therefore, an impact is applied to the upper thread when the upper thread is taken-up by the thread retainer of the thread take-up unit, so that the upper thread may be cut. Further, because the thread retainer of the thread take-up unit is moved backward the thread transferring path, much time is taken to replace the upper thread, thereby decreasing work efficiency. DISCLOSURE OF INVENTION Accordingly, it is an aspect of the present invention to provide a thread feeding apparatus for an automatic embroidering machine, which prevents an upper thread from being cut when the upper thread is replaced, and reduces time taken to replace the upper thread, thereby enhancing work efficiency. The foregoing and other aspects of the present invention are achieved by providing a thread feeding apparatus for an automatic embroidering machine, comprising a thread supplying unit to supply one upper thread selected among a plurality of upper threads different in color; a pair of thread guides formed with a thread guiding channel to guide the upper thread supplied from the thread supplying unit, and continuously formed along a thread transferring path of the thread guiding channel, leaving a space portion; a thread take-up unit provided with a thread retainer to retain the upper thread, oscillating between a top dead point and a bottom dead point, and strengthening and releasing the tension of the upper thread; and a thread take-out unit comprising a thread take-out lever to hold and take out the upper thread passing through the space portion, and a thread take-out driver to reciprocate the thread take-out lever to put the upper thread held by the thread take-out lever into the thread retainer. According to an aspect of the present invention, the thread take-out lever moves on a plane at a predetermined angle to the thread transferring path of the upper thread passing through the space portion, and puts the upper thread into the thread retainer by holding the upper thread passing through the space portion. According to an aspect of the present invention, the thread take-up unit comprises an arm oscillating between the top dead point and the bottom dead point; the thread retainer having an annular shape and formed with an opening, placed in a predetermined region of the arm, and retaining the upper thread; and an arm driver to drive the arm to oscillate. According to an aspect of the present invention, the thread take-up unit further comprises a flap that is rotatably coupled to the thread retainer, opens and closes the opening, and prevents the upper thread retained in the opening from a breakaway. According to an aspect of the present invention, the thread take-out unit comprises a bracket rotatably coupled to the thread take-out lever, and forming a flap accommodating portion to accommodate the flap therein, along with the thread take-out lever; and an elastic member rotating the bracket with predetermined elasticity to make the flap get out of the flap accommodating portion. According to an aspect of the present invention, the thread take-up unit further comprises a clamp that is elastically supported by the thread retainer, opens and closes the opening, and is made of an elastic material to prevent the upper thread retained in the opening from a breakaway. According to an aspect of the present invention, the clamp comprises a supporting portion supported by the thread retainer; a pressing portion longitudinally extended from the supporting portion, pressing a side of the thread retainer to prevent the upper thread retained in the thread retainer from a breakaway, and closing the opening; and an insertion portion bent and extended from one end of the pressing portion to be spaced from the side of the thread trainer and through which the thread take-out lever inserted. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial side-sectional view schematically illustrating a thread feeding apparatus for an automatic embroidering ' machine according to a first embodiment of the present invention. FIG. 2 is a partial enlarged perspective view of the thread feeding apparatus for the automatic embroidering machine of FIG. 1. FIG. 3 is an exploded perspective view of FIG. 2. FIG. 4 is a side view of FIG. 2. FIG. 5 is an exploded perspective view of a thread retainer of FIG. 2. FIGS. 6A through 6C are plan views illustrating an operating process of the thread retainer and a thread take-out lever. FIGS. 7A through 7D are side views illustrating an operating process of the thread retainer and the thread take-out lever. FIG. β is a partial enlarged perspective view illustrating that a thread is retained in the thread retainer when the thread retainer is placed at a bottom dead point. FIG. 9 is a partial enlarged perspective view illustrating that a thread is retained in the thread retainer when the thread retainer is placed at a top dead point. FIG. 10 is a partial side-sectional view schematically illustrating a thread feeding apparatus for an automatic embroidering machine according to a second embodiment of the present invention. FIG. 11A through lie are plan views illustrating an operating process of a thread retainer without a clamp and a thread take-out lever of FIG. 10. FIGS. 12A through 12D are side views illustrating an operating process of the thread retainer with the clamp and the thread take-out lever of FIG. 10. MODES FOR CARRYING OUT THE INVENTION Hereinbelow, a single needle automatic embroidering machine will be described as an embodiment of the present invention with reference to accompanying drawings, wherein like reference numerals refer to like elements throughout, and repetitive descriptions will be avoided as necessary. As shown in FIGS. 1 through 4, a thread feeding apparatus 1 for an automatic embroidering machine according to a first embodiment of the present invention includes a thread supplying unit 5 to supply one upper thread 3 selected among a plurality of upper threads different in color; a pair of thread guides 10a and 10b to guide the upper thread 3 supplied from the thread supplying unit 5 to a needle 9; a thread take-up unit 20 to take up the upper thread 3 passing through the pair of thread guides 10a and 10b, and strengthen and release the tension of the upper thread 3 ; and a thread take-out unit 40 to hold and take out the upper thread 3 passing through the pair of thread guides 10a and 10b, and put the upper thread 3 onto the thread take-up unit 20. The thread supplying unit 5 adjusts the tension of one upper thread 3 selected among the plurality of standby upper threads different in color, and employs compressed air for transferring the selected upper thread 3 to the pair of thread guides 10a and 10b. The thread guides 10a and 10b are provided with thread guiding channels 11a and lib to guide the upper thread 3 supplied from the thread supplying unit 5, respectively. Further, the thread guides 10a and 10b form a space portion 15 along a thread transferring path of the thread guiding channels 11a and lib. According to an embodiment of the present invention, the pair of thread guides 10a and 10b is formed as a single block formed with the space portion 15 and having a rectangular section. Hereinafter, the thread guide placed in an upper part will be called an upper thread guide 10a, and the thread guide placed in a lower part will be called a lower thread guide 10b. Preferably, the upper thread guide 10a and the lower thread guide 10b form a space portion 15 along the thread transferring path of the thread guiding channels 11a and lib, wherein the thread guiding channels 11a and lib are continuously aligned on the same axis. Alternatively, the thread guiding channels 11a and lib may be continuously and eccentrically aligned. Further, the upper thread guide 10a and the lower thread guide 10b may be formed as not the single block having the rectangular section but a pair of blocks which approach to and are spaced apart from each other to form the space portion 15. Also, the upper thread guide 10a and the lower thread guide 10b may have various section shapes, and may be either formed as a single tube-like body having the space portion 15 or a pair of tube-like bodies that approach to and are spaced from each other to form the space portion 15. The thread take-up unit 20 includes an arm 21 oscillating between a top dead point and a bottom dead point, a thread retainer 25 forming an opening 27 and provided in a predetermined region of the arm 21 so as to retain the upper thread 3, and an arm driver (not shown) to oscillate the arm 21. The arm 21 is driven by the arm driver to oscillate between the top dead point and the bottom dead point which are formed in front of the thread transferring path. Here, a hydraulic cylinder, a motor, a cam device, a solenoid device or the like can be used in the arm driver. The thread retainer 25 is provided in a free end of the arm 21, and oscillates between the top dead point and the bottom dead point along with the arm 21. As shown in FIG. 5, the thread retainer 25 forms the opening 27 to retain the upper thread 3 to be open at a side thereof. Here, the thread retainer 25 may have an annular shape, and may be provided in a predetermined side region of the arm 21. In addition, the thread take-up unit 20 of the thread feeding apparatus 1 for the automatic embroidering machine according to the first embodiment of the present invention further includes a flap 31 to prevent the upper thread 3 retained in the opening 27 of the thread retainer 25 from a breakaway. Here, the flap 31 is provided adjacent to an opened region of the opening 27, and opens and closes the opening 27. The flap 31 is rotatably coupled to the thread retainer 25. Further, the flap 31 has one side elastically supported by a flap elastic member 35 in the thread retainer 25. The flap elastic member 35 elastically urges the flap 31 to close the opening 27. According to an embodiment of the present invention, a coil spring is illustrated as the flap elastic member 35, but not limited to. Alternatively, a flat spring, a spiral spring, etc. as well as the coil spring can be used as the flap elastic member 35. Further, when the flap 31 is rotated by the flap elastic member 35 and closes the opening 27, the free end of the flap 31 is stopped by a stopper 29 provided in the opened region of the thread retainer 25, and does not rotate any more. Thus, the flap 31 opens and closing the opening 27 by rotating within the opening 27. Further, the flap 31 opens the opening 27 when a thread take-out lever 41 to be described later is pressed, and closes the opening 27 by the flap elastic member 35 to prevent the upper thread 3 retained in the opening 27 from a breakaway. Here, the flap 31 can be optically provided. The thread take-out unit 40 includes the thread take-out lever 41 to hold and take out the upper thread 3 passing through the space portion 15, and a thread take¬ out driver 51 to reciprocate the thread take-out lever 41. The thread take-out lever 41 linearly reciprocates along a plane at a predetermined angle to the thread transferring path of the upper thread 3 passing through the space portion 15. The thread take-out lever 41 includes a driver connection portion 43 connected to the thread take-out driver 51, and a pair of holders 45a and 45b lengthwise elongated leaving a predetermined space from a first side of the driver connection portion 43 and linearly reciprocating within the space portion 15. Hereinafter, the holder facing the upper thread guide 10a will be called an upper holder 45a, and the holder facing the lower thread guide 10b will be called a lower holder 45b. The upper and lower holders 45a and 45b together with a bracket 55 to be described later form a flap accommodating portion 61 to accommodate the flap 31. Further, the upper and lower holders 45a and 45b are respectively formed with thread through holes 47a and 47b through which the upper thread 3 is passed, thereby holding the upper thread 3 passing through the space portion 15. Thus, on the plane at a predetermined angle to the thread transferring path of the space portion 15, the thread take-out lever 41 linearly moves toward the thread retainer 25 placed at the bottom dead point, and holds the upper thread 3 passing through the space portion 15, thereby putting the upper thread 3 into the thread retainer 25. Alternatively, the thread take-out lever 41 may rotate along the plane at a predetermined angle to the thread transferring path of the upper thread 3 passing through the space portion 15, and hold the upper thread 3 passing through the space portion 15, thereby putting the upper thread 3 into the thread retainer 25. Further, the thread take-out lever 41 may curvedly move within a plane having a predetermined radius of curvature with respect to the thread transferring path of the upper thread 3 passing through the space portion 15, and hold the upper thread 3 passing through the space portion 15, thereby putting the upper thread 3 into the thread retainer 25. Instead of the thread through holes 47a and 47b, the respective free ends of the holders 45a and 45b may be opened and formed with semicircular-arc shaped holing grooves to take-up the upper thread 3 passing through the space portion 15, thereby putting the upper thread 3 into the thread retainer 25 of the thread take-up unit 20. Further, the holders may be formed as a single body to hold the upper thread 3 passing through the space portion 15 and putting the upper thread 3 into the thread retainer 25. The thread take-out driver 51 is connected to the driver connection portion 43 of the thread take-out lever 41, and reciprocates the thread take-out lever 41 to put the upper thread 3, which is held by the thread take-out lever 41 and passing through the space portion 15, into the thread retainer 25 of the thread take-up unit 20. In this embodiment, a hydraulic cylinder is used as the thread take-out driver 51, which is longitudinally extended and reduced to reciprocate the thread take-out lever 41, but not limited to. In addition to the hydraulic cylinder, a motor, a cam device, a solenoid device or the like can be used to driver the thread take¬ out lever 41. In the meantime, the bracket 55 shaped like a plate is rotatably connected by a pin 59 to the driver connection portion 43 of the thread take-out lever 41. The bracket 55 together with the thread take-out lever 41 • presses the flap 31, and opens the opening 27 of the thread take-up unit 20, thereby retaining the upper thread 3 within the opening 27. Further, a profile 57 having a predetermined shape is provided on the bracket 55 placed in a region of the holders 45a and 45b of the thread take-out lever 41. The profile 57 is in sliding contact with the flap 31. The profile 57 includes a leaner moving portion 57a formed in parallel with the linear moving direction of the holders 45a and 45b and linearly moving the flap 31 accommodated in the flap accommodating portion 61, and a transverse moving portion 57b transversely formed from one side of the liner moving portion 57a at a predetermined angle to the liner moving direction of the holders 45a and 45b. Further, one side of the bracket 55 is elastically supported by an elastic member 65. The elastic member 65 urges the bracket 55 to elastically press the flap 31 accommodated in the flap accommodating portion 61. In this embodiment, the flat spring is used as the elastic member 65, but not limited to. Alternatively, a coil spring, a spiral spring, or the like can be used as the elastic member 65. Thus, after the upper thread 3 is retained in the opening 27 of the thread take-up unit 20 by approaching the thread take-out lever 41 and the bracket 55 toward the thread retainer 25 of the thread take-up unit 20, when the thread take-out lever 41 and the bracket 55 are driven by the thread take-out driver 51 to be returned to an original position in the state that the flap 31 is accommodated in the flap accommodating portion 61, the flap 31 linearly slides along the transverse moving portion 57a of the profile 57 of the bracket 55 and then slides along the transverse moving portion 57b of the profile 57 in the state that the flap 31 closes the opening 27, so that the free end of the bracket 55 rotates to become gradually spaced from the thread take¬ out lever 41, thereby opening the flap accommodating portion 61 and allowing the flap 31 to get out of the flap accommodating portion 61 of the thread take-out unit 40. Meanwhile, the bracket 55 rotates toward the lateral side of the thread take-out lever 41 by the elasticity of the elastic member 65 and returns to its original position as soon as the flap 31 gets out of the flap accommodating portion 61. Here, the bracket 55 and the elastic member 65 may be optionally provided. With this configuration, when the upper thread is replaced as a color is changed while embroidering or the upper thread is cut, a process of putting a new thread into the thread take-up unit 20 using the thread feeding apparatus 1 according to the first embodiment of the present invention is as follows. First, the arm driver is driven to place the thread retainer 25 of the thread take-up unit 20 at the bottom dead point as shown in FIG. 4, and then the arm driver is stopped. At this time, the respective holders 45a and 45b of the thread take-out lever 41 is placed within the space portion 15, i.e., between the upper thread guide 10a and the lower thread guide 10b. Further, the thread guiding channels 11a and lib of the respective thread guides 10a and 10b are aligned with the thread through holes 47a and 47b of the respective holders 45a and 45b on the same axis. Further, the thread supplying unit 5 supplies one upper thread 3 selected among the plurality of standby upper threads different in color to the thread guiding channels 11a and lib of the respective thread guides 10a and 10b, using the compressed air. At this time, the upper thread 3 passes the thread guiding channel 11a of the upper thread guide 10a, the thread through holes 47a and 47b of the respective holders 45a and 45b of the thread take-out lever 41, and the thread guiding channel lib of the lower thread guide 10b in sequence, and then moves to the needle 9. Then, as shown in FIG. 6A, the thread take-out driver 51 is driven forward, so that the thread take-out lever 41 passing through the thread through holes 47a and 47b of the respective holders 45a and 45b and holding the upper thread 3 approaches to the thread retainer 25 of the thread take-up unit 20 placed at the bottom dead point. As the thread take-out driver 51 is driven forward, the thread take-out lever 41 together with the bracket 55 presses the flap 31 of the thread take-up unit 20, so that the flap 31 closing the opening 27 of the thread retainer 25 is inwardly rotated to open the opening 27 as shown in FIG. 6B. When the upper thread 3 held by the thread through holes 47a and 47b of the thread take-out lever 41 is retained in the opening 27 as shown in FIG. 7A, and then placed in a predetermined position within the opening 27 as the thread take-out driver 51 is further driven forward, the flap 31 is returned by the elasticity of the flap elastic member 35 to its original position, i.e., a position to close the opening 27. At this time, the flap 31 is accommodated in the flap accommodating portion 61 formed by the thread take¬ out lever 41 and the bracket 55, and is in contact with the profile 57 of the bracket 55. Then, the thread take-out driver 51 is driven backward to space the thread take-out lever 41 from the thread retainer 25 of the thread take-up unit 20. As the thread take-out driver 51 is driven backward, the flap 31 linearly slides along the linear moving portion 57a of the profile 57 of the bracket 55 and then approaches to the transverse moving portion 57b of the profile 57 in the state that the flap 31 closes the opening 27. Successively, the flap 31 slides along the transverse moving portion 57b as shown in FIG. 7B, so that the free end of the bracket 55 rotates to become gradually spaced from the thread take-out lever 41 and open the flap accommodating portion 61 as shown in FIG. 7C, thereby getting the flap 31 out of the flap accommodating portion 61 of the thread take-out unit 40. As soon as the flap 31 gets out of the flap accommodating portion 61, the bracket 55 rotates toward the lateral side of the thread take-out lever 41 by the elasticity of the elastic member 65 and returns to its original position as shown in FIG. 7D. Then, the thread take-out driver 51 is driven backward, so that the thread take-out lever 41 returns to its original position within the space portion 15 so as to align the thread transferring path of each thread guide 10a, 10b with the thread through holes 47a and 47b of the respective holders 45a and 45b, thereby completing the thread replacing work of the thread feeding apparatus 1 for the automatic embroidering machine according to the first embodiment of the present invention. At this time, as shown in FIG. 6C, the upper thread 3 passes through the thread guiding channel 11a of the upper thread guide 10a, and is turned as being caught by the flap 31 via the thread through hole 47a of the upper holder 45a. Then, the upper thread 3 moves toward the needle 9 through the thread guiding channel lib of the lower thread guide 10b via the thread through hole 47b of the lower holder 45b. Then, as shown in FIG. 9, the arm driver drives the thread retainer 25 of the thread take-up unit 20 to be placed at the upper dead point, so that the tension of the upper thread 3 retained in the thread retainer 25 is strengthened. Therefore, as the arm driver oscillates, the tension of the upper thread 3 retained in the thread retainer 25 is strengthened and released because the upper thread 3 oscillates between the bottom dead point and the top dead point. In the meantime, FIG. 10 is a partial side-sectional view schematically illustrating a thread feeding apparatus for an automatic embroidering machine according to a second embodiment of the present invention. In a thread feeding apparatus 1' for the automatic embroidering machine according to the second embodiment of the present invention, an upper holder 45a of a thread take-out lever 41' is formed with a thread through hole 47a, and a lower holder 45b is formed with an opened semicircular-arc shaped holding groove 47c, contrarily to the first embodiment. Here, the holding groove 47c of the lower holder 45b prevents the upper thread 3 from a breakaway and aligns the upper thread 3 when the upper thread 3 is retained in a thread retainer 25 of a thread take-up unit 20. Further, the thread retainer 25 having an annular shape of the thread take-up unit 20 is provided with a thread retaining groove 25a in which the upper thread 3 is retained and turned about in its direction. Also, the thread take-up unit 20 includes a clamp 71 which is elastically supported by one side of the thread retainer 25, made of elastic materials, opens and closes an opening 27 of the thread retainer 25, and prevents the upper thread 3 retained in the opening 27 from a breakaway. The clamp 71 includes a supporting portion 73 supported by the side of the thread retainer 25; a pressing portion 75 longitudinally elongated from the supporting portion 73 and closing the opening 27 by pressing the side of the thread retainer 25 so as to prevent the upper thread 3 retained in the thread retainer 25 from a breakaway; and an insertion portion 77 bent and extended from the end of the pressing portion 75 to be spaced from the side of the thread retainer 25 and through which the lower holder 45b of the thread take-out lever 41' is inserted. In this embodiment, the pressing portion 75 is lengthwise extended from one end of the supporting portion 73, having a half-wave shape. Thus, the end of the pressing portion 75 presses the side of the thread retainer 25. Preferably, a flat spring is employed as the clamp 71. With this configuration, a process of putting the upper thread 3 into the thread take-up unit 20 using the thread feeding apparatus 1' according to the second embodiment of the present invention is as follows. First, the arm driver is driven to place the thread retainer 25 of the thread take-up unit 20 at the bottom dead point, and then the arm driver is stopped. Further, the thread supplying unit 5 supplies one upper thread 3 selected among the plurality of standby upper threads different in color to the thread guiding channels 11a and lib of the respective thread guides 10a and 10b, using the compressed air. At this time, the upper thread 3 passes the thread guiding channel 11a of the upper thread guide 10a, the thread through holes 47a of the upper holder 45a and the holding groove 47c (refer to FIG. 12A) of the lower holder 45b of the thread take¬ out lever 41' in sequence, and then moves to the needle 9 via the thread guiding channel lib of the lower thread guide 10b. Then, as shown in FIGS. 11A and 12A, the thread take-out driver 51 is driven forward, so that the thread take-out lever 41' holding the upper thread 3 approaches to the thread retainer 25 of the thread take-up unit 20 placed at the bottom dead point. Meanwhile, to help a user's understanding of the shape of the respective holders 45a and 45b of the thread take-out lever 41', FIG. 12A illustrates the side view of the thread take-out lever along with its perspective view. As the thread take-out driver 51 is driven forward, the lower holder 45b of the thread take-out lever 41' is inserted in the insertion portion 77 of the clamp 71 of the thread take-up unit 20, so that the pressing portion 75 of the clamp 71 closing the opening 27 of the thread retainer 25 is spaced from the side of the thread retainer 25 and thus does not presses the side of the thread retainer 25 as shown in FIGS, lib and 12b, thereby opening the opening 27. At this time, the thread retainer 25 of the thread take-up unit 20 is placed between the pair of holders 45a and 45b. Then, as the thread take-out driver 51 is driven forward, the upper thread 3 held by the thread through hole 47a of the thread take-out lever 41' is retained within the opening 27. Then, when the upper thread 3 approaches to a predetermined position of the opening 27, i.e., when the upper thread 3 is placed in a position spaced from a pressing position of the pressing portion 75 of the clamp 71, the thread take-out driver 51 is driven backward to space the thread take-out lever 41' from the thread retainer 25 of the thread take-up unit 20. As the thread take-out driver 51 is driven backward, the lower holder 45b of the thread take-out lever 41' gets out of the pressing portion 75 and the insertion portion 77 of the clamp 71. As soon as the lower holder 45b gets out of the pressing portion 75, the pressing portion 75 presses the side of the thread retainer 25 by predetermined elasticity, thereby allowing the clamp 71 to return its original position to close the opening 27 of the thread retainer 25. At this time, the upper thread 3 passing through the thread through hole 47a of the upper holder 45a is retained in the thread retaining groove 25a of the thread retainer 25. Further, the pressing portion 75 of the clamp 71 prevents the upper thread 3 retained in the thread retaining groove 25a from getting out of the opening 27 of the thread retainer 25, and makes the upper thread 3 turn about in its moving direction. Then, the thread take-out driver 51 is driven backward, so that the thread take-out lever 41' returns to its original position within the space portion 15 in order to align the thread transferring path of each thread guide 10a, 10b with the thread through holes 47a of the upper holder 45a and the holding groove 47c of the lower holder 47b, thereby completing the thread replacing work of the thread feeding apparatus 1' for the automatic embroidering machine according to the second embodiment of the present invention. At this time, the upper thread 3 passes through the thread guiding channel 11a of the upper thread guide 10a, and is turned about its direction as being caught by the thread retaining groove 25a of the thread retainer 25 via the thread through hole 47a of the upper holder 45a. Then, the upper thread 3 moves toward the needle 9 through the thread guiding channel lib of the lower thread guide 10b via the holding groove 47c of the lower holder 45b. Thus, the upper thread passing through the space portion between the pair of thread guides to guide the upper thread supplied from the thread supplying unit is held, taken out, and put into the thread retainer of the thread take-up unit, so that excessive force is not applied to the upper thread, thereby preventing the upper thread from being cut when the upper thread is replaced. Further, the thread replacing work is performed in an operating region of the thread retainer of the thread take-up unit, so that the time taken to replace the upper thread is reduced, thereby enhancing work efficiency. In the meantime, it is to be easily understood by those skilled in the art that the invention is not limited to the single needle automatic embroidering machine, and can be applied to a multi-needle automatic embroidering machine. As described above, the present invention provides a thread feeding apparatus having an improved structure of feeing a thread take-up unit with an upper thread, so that the upper thread is prevented from being cut when the upper thread is replaced, and time taken to replace the upper thread is reduced, thereby enhancing work efficiency.