Abstract: An actuation member (B) supported by a housing (A) is actuated for clamping upwardly by means of a spring force and actuated downwardly for unclamping by means of an actuation means (D). A piston (11) is inserted into a cylinder (10) fixedly secured to the upper portion of the housing (A), and a liquid sealing chamber (12) is formed below the piston (11). The piston (11) is fixedly secured to the actuation member (B). When the actuation member (B) is held in the clamped condition, a liquid sealing valve (15) is closed so as to seal an oil (L) within the liquid sealing chamber (12). Whereby, in case that a pulling down force larger than the clamping spring force is imposed to the actuation member (B), a pressure generated within the liquid sealing chamber (12) serves to prevent the piston (11) from lowering, against the pulling down force.