To prevent the deformation of a work when the work is pulled apart from a clamp main body after releasing a clamp.

Under such a clamp state that a clamp arm 4 clamps a work 9, the piston 12 and piston rod 5 of a pneumatic cylinder 1 are retreated, as the pneumatic cylinder 1 being maintained to a prescribed position by a spring 20. Under such unclamp state the clamp arm 4 is clamp-released, the piston 12 is driven forward upto the place where it is regulated by a stopper rod 13 projected thereto and the piston rod 5 is advanced. When an air pressure is further added to the piston 12 from the unclamp state, the pneumatic cylinder 1 is retreated along with the clamp main body 2 along the guide rod 16 of a guide frame 11, as the piston 12 being regulated the moving against the guide frame 11 by the stopper rod 13.