To minimize an increase in sliding resistance and the generation of abnormal friction, by reducing bending moment or a radial offset load which a piston of an expander receives from a swash plate.

A spherical projecting portion 61a formed at a tip end of the piston 42 of axial piston cylinders abuts on a spherical recessed portion 31a formed on the swash plate 31, and an elliptic contact track T of a contact point (p) where the spherical recessed portion 31a and the spherical projecting portion 61a contact with each other is offset in an expanding stroke side of the axial piston cylinders. Thereby, in a middle range of the expanding stroke wherein the speed of the piston 42 is increased and surface pressure at the contact point (p), the position of the contact point (p) is made to be close to an axis line L3 of the spherical recessed portion 31a and an axis line L2 of the piston 42. The bending moment or the radial offset load acted on the piston 42 is reduced, thereby minimizing the increase in sliding resistance or the generation of abnormal friction.