PURPOSE: To stabilize the performance so that no influence is given to the temperature variation, as for a check valve using a shaft-shaped member.

CONSTITUTION: A check valve 4 which permits the flow of air into an atmosphere chamber 1a from a working pressure chamber 1b and suppresses the flow of working oil is installed on a piston 2, and the check valve 4 is constituted of a cylindrical valve cylinder 41 fitted with a through hole 2b which permits the communication between the atmosphere chamber 1a and the working pressure chamber 1b and penetrates through the piston 2 and a shaft-shaped member 42 which is inserted into a fine gap formed and a set between the inner periphery of the valve cylinder 41 and the shaft-shaped member 42, and the valve cylinder 41 and the shaft-shaped member 42 are made of the material having the nearly equal thermal expansion coefficient.