To prevent, during switching, a traverse width from being larger than a traverse width during normal high-speed traversing by reducing the tension width of yarn with a traverse width regulation guide while reducing a traverse speed.

A member L-shaped in section is supported in a position opposite the right side face of the supporter 3 of a machine frame 1 so as to be swung around its upper end, and a traverse width regulation guide 28 is formed in its lower part tip. By swinging this member L-shaped in section, the traverse width regulation guide 28 is moved from a standby position opposite a traverse surface 37 to the vicinity of the end part of a yarn traverse width on the traverse surface. Then, it is moved to a position for regulating a traversed yarn traverse stroke width. A recessed part 28c corresponding to a traverse stroke is formed in the plate-like member of the traverse width regulation guide 28. Thus, during yarn switching, the increase of the traverse width caused by the reduction of a traverse speed is prevented.