PURPOSE: To eliminate the generation of a skip stitch even at high speed sewing with a simple structure and allow the sewing with a low tension multi-thread chain looper thread by forming a multi-thread chain looper thread locking structure on one side of the inter side needle groove of a throat plate.

CONSTITUTION: When a multi-thread chain looper 11 is retreated to an initial position, the base part of a thread delivered from the lower surface of a needle groove 2 after scooping up a multi-thread chain looper back triangular thread (a') is released from a multi-thread chain looper thread locking recessed groove 22 to extinguish the multi-thread chain looper back triangular thread (a'), and one stitch is formed. The multi-thread chain looper back triangular thread (a') formed under a throat plate 1 is temporarily engaged with the multi-thread chain looper thread-locking recessed channel 22 of the throat plate 1, and the needle fall effective area of an inter needle 10 is forcedly extended to solve a skip stitch, so that the tension of a multi-thread chain looper thread (a) can be sufficiently lowered. Thus, no puckering is caused even in the case of a thin cloth, and the most preferable seam in which the multi-thread chain looper thread (a) is curved in a balloon can be formed.