PURPOSE: To release biting of a wedge shaft to a work receiver through shocking force, by fitting a free piston to a fluid pressure piston driving the wedge shaft in a work supporter using a wedge shaped clamp used for machining work.

CONSTITUTION: If air is fed from an air inlet outlet port 21 of a cylinder head 23, a wedge shaft 4 advances to lift a work receiver 3 and forcibly contact the receiver to a work W then continuously support the work W. To release supporting of the work W, a feed of air from the port 21 is stopped, if air is fed from an air inlet outlet port 22 in the root of a fluid pressure cylinder 17, a free piston 18 is moved backward by air pressure applied to the front face and accelerated to collide against a stopper 20. A shock of this collision releases biting of the shaft 4 to the receiver 3.