FIELD OF THE INVENTION The present invention relates to a sewing machine, and more particularly, to a sewing machine capable of cutting a thread snapped while performing an embroidering work, thereby returning the thread to a needle working unit. BACKGROUND ART As a kind of a sewing machine, an automatic embroidering machine has been widely known, which uses various colored threads and automatically embroiders a predetermined pattern, a trademark, or the like on clothes, a label of shoes, etc. according to a preset program. In general, the automatic embroidering machine includes a head stem placed in front of a frame; an upper thread supplying unit mounted to the head stem and supplying various colored upper threads from a plurality of bobbins toward an embroidering work place; and a needle working unit threading the upper thread supplied by the upper thread supplying unit and performing an embroidering work. The automatic embroidering machine is classified into a single-needle automatic embroidering machine and a multi-needle automatic embroidering machine according to the number of needles mounted to the upper thread supplying unit and the needle working unit. As an example of the sewing machine, the single- needle automatic embroidering machine will be described hereinafter. The conventional single-needle automatic embroidering machine has been disclosed in Korean Patent First Publication No. 2002-0068028, which is titled as "THREAD FEEDING APPARATUS FOR AUTOMATIC EMBROIDERING MACHINE." Such conventional single-needle automatic embroidering machine includes a head stem having a needle working unit; a thread supplying unit supplying one selected among a plurality of threads different in color; a guide plate assembly provided in the head stem and guiding the thread received from the thread supplying unit toward the thread-working unit; and a thread take-up unit strengthening and releasing the tension of the thread, so that the head stem is unified and a thread supplying path excludes a curved path and allows the thread to be smoothly supplied. Further, the conventional single-needle automatic embroidering machine includes a thread standby length adjuster provided between the bobbin and the thread supplying unit and maintaining and releasing a standby state of the thread. However, in the conventional single-needle automatic embroidering machine, when the thread connected to the needle working unit is snapped during the embroidering work, the end of the snapped thread untwists, so that it is not easy to thread a needle. Therefore, a worker should clearly cut the untwisted end of the thread, thereby lowering work efficiency. Further, in order to move the cut thread to the needle working unit, a worker has to manually place the cut thread to a standby position of the thread supplying unit, thereby decreasing work efficiency. DISCLOSURE OF INVENTION Accordingly, it is an aspect of the present invention to provide a sewing machine, in which a thread snapped during an embroidering work is automatically cut, thereby enhancing work efficiency. The foregoing and other aspects of the present invention are achieved by providing a sewing machine with a thread supplying unit for supplying a thread to a needle working unit, the sewing machine, comprising a cutting unit comprising a cutter to cut the thread, and a cutter supporter to support the cutter, and cutting the thread transferred from the thread supplying unit to the needle working unit along a thread transferring path; and a cutting driver driving the cutting unit to cut the thread on the thread transferring path by the cutter. According to an aspect of the present invention, the sewing machine further comprises a thread standby length adjuster provided between a bobbin and the thread supplying unit, and returning the cut thread to the thread supplying unit for transferring the cut thread from the bobbin to the needle working unit. According to an aspect of the present invention, the cutting driver comprises a supporting shaft supporting the cutter supporter; and a driving unit connected to the supporting shaft, and driving the cutter to contact and be spaced apart from the thread and to cut the thread on the thread transferring path. According to an aspect of the present invention, the cutting driver comprises a guide movably coupled to the supporting shaft, guiding the thread on the thread transferring path to a predetermined cutting position, and formed with a cutter through portion through which the cutter passes; and an elastic member provided between the cutter supporter and the guide, and elastically urging the cutter supporter to be spaced from the guide at a predetermined distance, wherein the driving unit drives the guide to guide the thread on the thread transferring path to the cutting position. According to an aspect of the present invention, the guide comprises a magnet. According to an aspect of the present invention, the thread standby length adjuster comprises a thread standby length adjusting body comprising a thread guiding unit to guide the cut thread wound on the bobbin to be transferred from the bobbin to the thread supplying unit, and a plurality of thread accommodating units communicating with the thread guiding unit and formed with an accommodating space to accommodate the cut thread on standby; a thread standby length adjusting bar provided movably along the thread accommodating unit, and returning a cut end of the thread to the thread supplying unit by transferring the cut thread directing to the bobbin from a working state in the thread guiding unit to a standby state. According to an aspect of the present invention, the thread standby length adjuster further comprises a thread transferring interceptor provided between the bobbin and the thread standby length adjusting body, and intercepting the thread transferred from the bobbin to the thread standby length adjusting body. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial perspective view of a sewing machine according to an embodiment of the present invention; FIG. 2A is a partial enlarged perspective view of a thread standby length adjusting bar of FIG. 1; FIG. 2B is a partial enlarged perspective view of a thread transferring interceptor; FIGS. 3A through 5B are partial enlarged perspective views and partial sectional views, which illustrate operations of a cutting unit and a cutting driver in a thread cutting process; and FIGS. 6 through 10 are sectional views illustrating the thread cutting process of FIG. 1 in sequence. 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. As shown in FIG. 1, a sewing machine 1 according to an embodiment of the present invention includes a frame 5, a head stem 7 placed in front of the frame 5 and mounted with a needle working unit (not shown) at a lower part thereof, a thread supplying unit 11 mounted to the head stem 7 and supplying one selected among a plurality of threads from a plurality of bobbins (not shown) different in color to the needle working unit, a cutting unit 31 to cut a thread 3 transferred to the needle working unit from the thread supplying unit 11 along a thread transferring path, a cutting driver 41 driving the cutting unit 31 to make a cutter 33 cut the thread 3 on the thread transferring path, and a thread standby length adjuster 71 returning the cut thread 3 from the bobbin to the thread-working unit. Further, the sewing machine 1 according to an embodiment of the present invention includes a threading unit (not shown) putting the thread 3 supplied by the thread supplying unit 11 to a needle (not shown) of the needle working unit, a thread take-up unit 9 taking up a part of the thread 3 transferred from the thread supplying unit 11 to the needle working unit so that the tension of the thread 3 is strengthened and released, and a thread tension adjuster (not shown) adjusting the tension of the thread 3 supplied to the needle working unit. The frame 5 is seated on a horizontal surface such as the ground, or a wall, or coupled to other machines, thereby supporting the head stem 7 or the like. The head stem 7 is approximately shaped like a rectangular plate to support the thread supplying unit 11, the needle working unit, the threading unit, etc. in front thereof. Alternatively, the head stem 7 may have various shapes to support the thread supplying unit 11 or the like. The thread supplying unit 11 includes a thread supplying block 13 having a plurality of thread standby pipes 15 in which an end of each thread 3 transferred from the plurality of bobbins is accommodated on standby, a plurality of connecting pipes 17 placed under the thread supplying block 13 and communicating with the respective thread standby pipes 15, one thread guiding pipe 19 (refer to FIG. 3B) communicating with the plurality of connecting pipes 17, a supporting block 21 supporting the plurality of connecting pipes 17 and the thread guiding pipe 19, and an air supply (not shown) connected to and communicating with the plurality of thread standby pipes 15 and selectively blowing high pressure air to selectively supply one among the plurality of threads 3 accommodated in the plurality of thread standby pipes 15 to the needle working unit. The supporting block 21 is provided with a cutter accommodating groove 23 at a front surface thereof, in which the cutter accommodating groove 23 is recessed in a predetermined depth transversely to the thread transferring path. Through the cutter accommodating groove 13, the thread 3 passes from the thread supplying unit 11 to the needle working unit along the thread transferring path. Therefore, the end of the cutter 33 of the cutting unit 31 (to be described later) is accommodated in the cutter accommodating groove 23, thereby contacting and cutting the thread 3 transferred along the thread transferring path. Preferably, the thread supplying unit 11 further includes an auxiliary air supply (not shown) coupled to the supporting block 21 and facilitating the thread 3 transferred through one of the plurality of connecting pipes 17 to be easily supplied to the needle working unit via the supporting block 21 and the thread guiding pipe 19. Meanwhile, the cutting unit 31 is disposed in front of the thread supplying unit 11 and moves close to and apart from the supporting block 21 of the thread supplying unit 11. Further, the cutting unit 31 includes the cutter 33 to cut the thread 3, and a cutter supporter 35 to support the cutter 33. The cutter 33 has a rectangular-shaped section, and is supported on the cutter supporter 35 while a blade thereof faces toward the supporting block 21. The cutting driver 41 includes a supporting shaft 43 to support the cutter supporter 35, and a driving unit 45 connected to the supporting shaft 43 and driving the cutter 33 to contact and be spaced apart from the thread 3, thereby cutting the thread 3 transferred along the thread transferring path. The driving unit 45 includes a hydraulic cylinder 47, and a cylinder rod 49 reciprocated by expanding and retracting due to an operation of the hydraulic cylinder 47. Preferably, the foregoing cylinder device is used as the driving unit 45, but not limited to. Alternatively, the driving unit 45 may includes a belt and a driving motor for the reciprocating operation. The cylinder rod 49 is linked to the supporting shaft 43 by a link 51. The link 51 transfers the reciprocating operation of the cylinder rod 49 to the supporting shaft 43, so that the cutter supporter 35 coupled to one end of the supporting shaft 43 is moved close to and apart from the thread 3 on the thread transferring path. Further, the cutting driver 41 includes a guide 53 movably coupled to the supporting shaft 43 and guiding the thread 3 on the thread transferring path to a predetermined cutting position, and an elastic member 61 elastically urging the cutter supporter 35 to be spaced from the guide 53 at a predetermined distance. The guide 53 is movably coupled to the supporting shaft 43, leaving a predetermined space from the cutter supporter 35. The guide 53 is formed with a cutter through hole 55 through which the cutter 33 passes to cut the thread 3. Further, the guide 53 is provided with a magnet 57 to contact the supporting block 21 of the thread supplying unit 11 by a magnetic force, so that the thread 3 on the thread transferring path firmly contacts a predetermined cutting position, i.e., a circumference of the cutter accommodating groove 23 of the supporting block 21, and at the same time tenses the thread 3 when the thread 3 is cut, thereby allowing the thread 3 to be easily cut. Further, the guide 53 is connected to a rotation arm 59 converting the linear reciprocation of the cylinder rod 49 into rotation at a predetermined position. The rotation arm 59 has a first end coupled to the guide 53, and a second end rotatably coupled to the head stem 7. The elastic member 61 is provided along the circumference of the supporting shaft 43 between the cutter supporter 35 and the guide 53. In this embodiment, a coil spring is used as the elastic member 61, but not limited to. Alternatively, a flat spring, a spiral spring, etc. can be employed as the elastic member 61. Thus, the driving unit 45 not only drives the guide 53 to guide the thread 3 on the thread transferring path to the circumference of the cutter accommodating groove 23 of the supporting block 21, but also drives the cutter 33 to be accommodated in the cutter accommodating groove 23 while contacting the thread 3 transverse to the cutter accommodating groove 23, thereby cutting the thread 3. In the meantime, the thread standby length adjuster 71 includes a thread standby length adjusting body 73 formed with an accommodating space to accommodate the plurality of threads 3 on standby between the bobbin and the thread supplying unit 11, and a thread standby length adjusting bar 81 to move the plurality of threads 3 to the accommodating space. Further, the thread standby length adjuster 71 includes a thread transferring interceptor 91 provided between the bobbin and the thread standby length adjusting body 73 and intercepting the plurality of threads 3 transferred to the thread standby length adjusting body 73. The thread standby length adjusting body 73 includes a plurality of thread guiding units 75 guiding the plurality of threads 3 transferred from the bobbin toward the thread supplying unit 11, and a plurality of thread accommodating units 77 formed with the accommodating space communicating with the respective thread guiding units 75 and accommodating the plurality of threads 3 on standby. Further, the thread standby length adjusting body 73 includes a guide slit 79 communicating with the thread accommodating unit 77 in a direction transverse to the thread transferring path. Preferably, the thread standby length adjusting body 73 is approximately shaped like a rectangular block. Preferably, the thread guiding unit 75 is recessed at a predetermined depth in a direction of transferring the thread 3 to separately accommodating the plurality of threads 3 on a top surface of the thread standby length adjusting body 73. The depth of the thread guiding unit 75 is determined to prevent the thread 3 from breakaway while the thread 3 is transferred. Further, the thread guiding unit 75 guides the cut thread 3 wound on the bobbin to be transferred from the bobbin to the thread supplying unit 11. The thread accommodating unit 77 is recessed downward at a predetermined depth from a bottom surface of each thread guiding unit 75, and has a predetermined width in the thread transferring direction. Preferably, the thread accommodating unit 77 is plurally provided along the thread transferring direction. According to an embodiment of the present invention, three thread accommodating units 77 are provided along the thread transferring direction. Further, the depth of the thread accommodating unit 77 is determined according to the standby length of the thread 3, and the standby length of the thread 3 is determined enough to make the end of the thread 3 accommodated in the thread standby pipe 15 of the thread supplying unit 11 be supplied toward the needle working unit and connected to an eye of the needle. Further, when the thread accommodating unit 77 is plurally provided, the standby length of each thread 3 is distributed and accommodated in each thread accommodating unit 77, so that the depth of each thread accommodating unit can be reduced. The guide slit 79 is provided to communicate with the thread accommodating unit 77, and accommodates and guides the thread standby adjusting length bar 81. Preferably, the guide slit 79 is formed downward along the thread accommodating unit 77 in a direction transverse to the thread transferring direction. Further, the guide slit 79 is plurally provided corresponding to the number of the thread accommodating units 77 provided in the thread transferring direction. According to an embodiment of the present invention, two thread accommodating units 77 are provided in the thread transferring direction, so that two guide slits 79 are provided correspondingly. The thread standby length adjusting bar 81 is movably accommodated in the guide slit 79, and transfers the plurality of threads 3 to a standby state accommodated in the thread accommodating unit 77 from a working state placed in the thread guiding unit 75 and moving to the needle working unit. Further, the thread standby adjusting bar 81 transfers the cut thread 3 directing to the bobbin from the working state in the thread guiding unit 75 to the standby state, thereby returning the cut end of the thread 3 to the thread supplying unit 11. Preferably, the thread standby length adjusting bar 81 is placed above the plurality of threads 3, and plurally provided corresponding to the number of guide slits 79 provided in the thread transferring direction. In this embodiment, two guide slits 79 are provided in the thread transferring direction, so that two thread standby length adjusting bars 81 are provided correspondingly. Further, the thread standby adjusting bar 81 is preferably provided with an adjusting bar supporter 83 at one side thereof, in which the adjusting bar supporter 83 is integrally formed with or coupled to the plurality of thread standby length adjusting bars 81. As shown in FIG. 2A, the adjusting bar supporter 83 is coupled to an adjusting bar driver 85 driving the thread standby length adjusting bar 81 to move along the glide slit 79. Preferably, the adjusting bar driver 85 is a cylinder device coupled to the adjusting bar supporter 83 and driving the adjusting bar supporter 83 to reciprocate in a vertical direction, but not limited to. Instead of the cylinder device, a belt and a driving motor may be used so that they are coupled to the adjusting bar supporter 83 and driving the adjusting bar supporter 83 to reciprocate in the vertical direction. Further, the thread standby length adjuster 71 includes the tread transferring interceptor 91 provided between the bobbin and the thread standby length adjusting body 73 and intercepting each thread 3 transferred from the bobbin to the thread standby length adjusting body 73. The thread transferring interceptor 91 is placed behind the thread standby length adjusting body 73. As shown in FIG. 2B, the thread transferring interceptor 91 includes a supporting bracket 93 mounted on the thread standby length adjusting body 73, and the supporting bracket 93 is formed with a through hole 93a through which the plurality of threads 3 transferred from the bobbin passes. The supporting bracket 93 is provided with an interceptor 95 at an top side, the interceptor 95 contacting and being spaced apart from the plurality of threads 3 transferred to the thread guiding unit 75 through the through hole 93a, and intercepting the plurality of threads 3 transferred to the thread guiding unit 75. The interceptor 95 is coupled to the supporting bracket 93 and can be lifted up and down. Here, the interceptor 95 is connected to an interceptor driver 101 by a pair of rods 97 and a connecting bar 99. The interceptor driver 101 is preferably a cylinder device coupled to the connecting bar 99 and lifting the interceptor 95 up and down toward the top surface of the supporting bracket 93. That is, the interceptor driver 101 drives the interceptor 95 to contact or be spaced apart from the plurality of threads 3 transferred to the thread guiding unit 75 along the top surface of the supporting bracket 93, thereby intercepting or allowing the plurality of threads 3 transferred to the thread guiding unit 75. With this configuration, the sewing machine 1 according to an embodiment of the present invention cuts the thread 3 as follows. In the case where the thread 3 connected to the needle working unit is cut during the embroidering work, the embroidering work is stopped. At this time, as shown in FIGS. 3A, 3B and 6, the cut thread 3 transferred from the thread supplying unit 11 to the needle working unit along the thread transferring path is retained on the thread take-up unit 9, and the cut end of the thread 3 is untwists so that it is not easy to thread the needle of the needle working unit. Then, the cylinder rod 49 of the driving unit 45 is retracted, so that the cutter 33 of the cutting unit 31 is moved to the cutting position, i.e., the cutting accommodating groove 23 of the supporting block 21 (refer to FIGS. 4A and 4B) . As the cylinder rod 49 of the driving unit 45 is retracted, the link 51 moves toward the driving unit 45, and the supporting shaft 43 is rotated by the rotation arm 59 and disposed on the same plane as the cylinder rod 49. At this time, the guide 53 is rotated toward the supporting block 21 as the supporting shaft 43 rotates, thereby making the thread 3 retained on the thread take- up unit 9 be in contact with the front surface of the supporting block 21. Further, when the cylinder rod 49 is more retracted, the cutter supporter 35 moves toward the guide 53 in the state that the guide 53 is in contact with the front surface of the supporting block 21. At this time, as shown in FIGS. 5A, 5B and 7, the edge of the cutter 33 passes through the cutter through hole 55 of the guide 53 and contacts and cuts the thread 3 disposed transversely to the cutter accommodating groove 23. At the same time, the elastic member 61 interposed between the cutter supporter 35 and the guide 53 is compressed. Here, the end of the cut thread 3 is clearly cut enough to be easily put to the needle of the needle working unit. After cutting the thread 3, as shown in FIG. 8, the adjusting bar driver 85 is driven to make the thread standby length adjusting bar 81 move down along the guide slit 79 of the thread standby length adjusting body 73 while the guide 53 is in contact with the front surface of the supporting block 21, thereby accommodating the cut thread 3 from the bobbin in the thread accommodating unit 77. At this time, the thread transferring interceptor 91 allows the thread 3 to be transferred. After the cut thread 3 is supplied from the bobbin to the thread accommodating unit 77 by a predetermined length, the interceptor driver 101 drives the thread 3 to be not transferred from the bobbin to the thread standby length adjusting body 73. Then, as shown in FIG. 9, while the thread 3 is not transferred from the bobbin to the thread standby length adjusting body 73, the cylinder rod 49 of the driver 45 is expanded, thereby spacing the guide 53 from the supporting block 21. At this time, the thread 3 retained on the thread take-up unit 9 and the thread 3 transferred from the thread take-up unit 9 to the needle working unit get out of the sewing machine 1. Then, in order to return the cut thread 2 extended between the cutter accommodating groove 23 and the bobbin to the needle working unit, the thread standby length adjusting bar 81 is more moved down along the guide slit 79 of the thread standby length adjusting body 73, thereby returning the end of the cut thread 3 to the standby position of the thread supplying unit 11, i.e., placing the end of the cut thread 3 in the connecting pipe 17 of the thread supplying unit 11. Thus, the end of the thread 3 snapped during the embroidering work is clearly cut enough to easily thread the needle of the needle working unit, and then returns to the thread standby position of the thread supplying unit 11, thereby completing preparation to supply the thread 3 to the needle working unit. After completing the preparation, the air supply blows the high pressure air so as to transfer the thread 3 accommodated in the connecting pipe 17 toward the needle working unit. Then, the threading unit (not shown) puts the thread 3 to the needle of the needle working unit, thereby making a sewing work possible. Thus, the thread snapped during the embroidering work is automatically cut by the cutter, so that work efficiency is enhanced. Further, the cut thread is automatically returned to the thread standby position of the thread supplying unit in order to returning the cut thread to the needle working unit, thereby enhancing work efficiency. Meanwhile, it will be appreciated by those skilled in the art that the invention can be applied to the single needle automatic embroidering machine, a multi- needle automatic embroidering machine, a sewing machine, etc. As described above, the present invention provides a sewing machine, in which a thread snapped during an embroidering work is automatically cut, thereby enhancing work efficiency.