PURPOSE: To avoid disorder of thread tension and breakage of a thread by providing a sub-thread tension part, a main thread tension part and a space wherein a sewing thread between the sub-thread tension part and the main thread tension part is not pinched and enlarging an opening part on each projected part of the main thread tension part and the sub-thread tension part toward a thread feeding side from a thread drawing side.

CONSTITUTION: A main thread tension part 14B and a sub-thread tension part 14A are respectively provided on a thread drawing side B and a thread feeding side A on a pair of thread tension trays 14 and 14' and an opening part of a crescent projected part 14b is provided around an axial hole 14b of the main thread tension part 14B in the entering direction of a sewing thread. In addition, a crescent projected part 14a is provided around an axial hole 14a' of the sub-thread tension part 14A in the same direction as the crescent projected part 14b. The crescent projected parts 14a and 14b are formed each by a gently-sloping slant from the upper face (d) of the projected part toward the lower face (e) and the sewing thread can be moved from a pinching part formed by the crescent projected parts 14 and 14' of the thread tension trays 14 and 14' toward a supporting shaft 13 and a thread tension shaft 12. A space wherein the sewing thread is not pinched is provided between the crescent projected parts 14a and 14b.