A communication path that communicates with a first main pressure chamber is provided in a main piston and a piston rod. A check valve, which opens by being pressed by a booster piston and allows the communication path to communicate with a first sub-pressure chamber when the piston rod reaches a booster start position before a forward stroke end, is disposed in an end portion of the communication path. A plurality of steel balls are disposed in a coupling-member containing chamber formed in the booster piston. An engagement surface and an engagement groove, which engage with the steel balls when the booster piston moves forward due to an action of a pressure fluid supplied to the first sub-pressure chamber 11a through the communication path, are formed in the coupling-member containing chamber and an outer peripheral surface of the piston rod.