A first clamping arm mounted to an arm rotary shaft is rotated by a driving mechanism formed of a worm wheel mounted to the arm rotary shaft, a worm supported on a junction frame, and a first driving source to clamp a workpiece between a second clamping arm and the first clamping arm. A rotating force in a direction opposite to a reaction force in clamping is applied by a clamping force applying mechanism to the junction frame which has been displaced through a certain angle by the reaction force to thereby generate a rotating force in a clamping direction in the arm rotary shaft through the worm and the worm wheel to thereby apply a clamping force to the first clamping arm.