To solve the problem that a front and back gripper rotating mechanism is complicated, and that a power factor of a force transmitting mechanism grows worse.

In the front and back gripper rotating mechanism, a link 27 for a back gripper is swung in accordance with changing in the outer diameter of a cam 25 by using a link supporting shaft 30b as a center of rotation and pushing a cam follower 29b set at a part of the link 27 on a back gripper opening/closing cam 25. A front gripper shaft 48 is similarly rotated by a gear 47 fixed on the front gripper shaft 48 and mating a gear part 31a of a link 28 for the front gripper swung by a cam follower 29a pushed on and come into contact with a front gripper opening/closing cam 26. In this case, the front gripper shaft 48 is hollow and is combined with a torsion bar 44 inside.