To easily provide desired tensile force uniform in left and right to a belt by laying an endless belt on a first shaft supported on a shaft bracket to be freely rotated and disposed perpendicularly to the extended direction of a worm rack and a second shaft placed in parallel with the first shaft.

For providing tension to a belt 9, a worm gear 1 is rotated by rotating a knob 5 with a hand, a shaft bracket 4 provided integrally with a worm rack 3 is moved backward by moving backward the worm rack 3 engaged with the worm gear 1, and then a rear shaft 7 is moved away from a front shaft 8. Thus, tension is provided to the belt 9 laid between the rear shaft 7 and the front shaft 8. When the moving amount of the worm shaft 3 exceeds a fixed value, the rear end part of the worm rack 3 is brought into contact with the rear end part of a worm base 2 and stopped, and thus the excessive increase of tension applied to the belt 9 is prevented.