A first piston (10) is inserted in a housing (2) so as to be hermetically movable vertically. A second piston (20) is inserted in a cylindrical hole (14) of the first piston (10) so as to be hermetically movable axially, and has an output rod (26) projecting upward from the housing (2). A lock chamber (40) is formed below the first piston (10) and the second piston (20), and a release chamber (42) is formed above the first piston (10) and the second piston (20). A force multiplier (52) is arranged in the housing (2) so as to cause (i) a first force, with which a pressurized fluid supplied into the lock chamber (40) pushes the first piston (10) upward, to be transformed in a force-multiplying manner into a second force and (ii) the second force to be transmitted to the second piston (20). Further, a change-over mechanism (54) is arranged in the housing (2) so as to selectively couple the first piston (10) to the housing (2) or the second piston (20).