PROBLEM TO BE SOLVED: To suppress an airtightness defect caused by inclination of a piston while securing a smooth operation, and to improve durability and a service life.SOLUTION: On a side face of a shaft body part 51b of a piston 51, a plurality of packing pieces 74a-74d are fitted in order to switch communication and blocking of a plurality of conduits. Among the packing pieces 74a-74d, end packing pieces 74a and 74d arranged at an axial direction end part of the shaft body part 51b are composed of fluororubber, and are composed of a material having higher wear resistance than that of silicone rubber configuring intermediate packing pieces 74b and 74c arranged at parts other than the axial direction end part. Thus, the smooth operation is made possible, the airtightness defect caused by the inclination of the piston 51 is suppressed, and the durability and the service life are improved.
