An output rod (27) is inserted in a housing (4) so as to be movable vertically. A first piston (41) inserted in a lower part of the housing (4) is fixed to the output rod (27). A second piston (42) inserted on an upper side of the first piston (41) is fitted on the output rod (27) so as to be movable vertically. A lock chamber (44) is provided between the first piston (41) and the second piston (42). A first release chamber (51) is provided below the first piston (41). A second release chamber (52) is provided above the second piston (42). During force multiplication driving of the output rod (27), a pressurized fluid in the lock chamber (44) carries out the force multiplication driving with respect to the output rod (27) downward via the second piston (42) and engaging balls (69). A retaining spring (60) which biases the second piston (42) upward is provided in the lock chamber (44) between the first piston (41) and the second piston (42).