A part of a rubber layer of the base section of a piston seal is replaced with a reinforcement ring with high rigidity, and a part of the reinforcement ring reaches the root of a seal lip. This provides the piston seal with required rigidity. Further, providing an outer diameter side radial section on a piston body secures a contact width coming into contact with a section to be pressed. A piston has a piston body (111) having a clutch pressing section (111f) provided so as to be in surface-contact with a multiple disc clutch (105), a reinforcement ring (112) fitted on the outer periphery of an outer cylinder section (111e) of the piston body, and an outer peripheral seal lip (113a) provided on the reinforcement ring and in sealing contact with the inner peripheral surface (102a) of the outer peripheral section of a housing (102).