Abstract: At a contact point, a spherical projecting portion of a piston presses a spherical recessed portion of a swash plate due to a working medium supplied to an operating chamber. The contact point is located on a cross line, on which a plane including a center of curvature of the spherical recessed portion and an axis Ls of the swash plate intersects the spherical recessed portion. Therefore, even if the piston forcefully presses the swash plate, the spherical projecting portion and the spherical recessed portion at the contact point are prevented from slipping in a circumferential direction, a radial direction and the like. Thus, the phases of a rotor and the swash plate are prevented from being displaced and causing vibration and noise. A large side thrust is prevented from acting on the piston and causing heat due to twisting and sliding friction between the piston and the cylinder.