Abstract: An expander is provided in which spherical projections (61a) formed at the forward end of pistons (42) of an axial piston cylinder group abut against spherical depressions (31a) formed in a swash plate (31), and an elliptical contact point locus (T) of contact points (p) between the spherical depressions (31a) and the spherical projections (61a) is offset toward the expansion stroke side of the axial piston cylinder group. As a result, in a middle region of the expansion stroke where the speed of the piston (42) is high and the surface pressure of the contact point (p) is high, the position of the contact point (p) can be made as close as possible to an axis (L3) of the spherical depression (31a) and an axis (L2) of the piston (42), thus reducing a bending moment and a radially biased load exerted on the piston (42) and minimizing any increase in sliding resistance and any occurrence of abnormal wear.