To prevent the production of defective products having damage or twists by detecting the removal of a wire from a pulley at an early time.

A tension measuring section is provided with an L-shaped arm 6, a potentiometer 7, a tensile spring 9, a pulley 2 having recesses 2b1 and 2b2, a nut 5 having a part of a tapered outer peripheral surface 5a fitted in the recess 2b1 of the pulley and attached to a pulley shaft 3, and a pulley fixing plate 4 having the tip of a slope 4a fitted in the recess 2b2 of the pulley and attached to the other end 6b of the L-shaped arm 6. A part of the L-shaped arm 6 from the other end 6b to a bent part 6c is arranged nearly in parallel with a straight line connecting the middle point O1 of a contact place between a wire 1 and the pulley 2 with the center O2 of the pulley.