To arbitrarily set a relationship between the strokes of a piston, with the rotation angle of a rotor of a rotation fluid machine.

A cam surface 25a, which changes its height in the axis L direction, is formed at a cam member 25 fixed to a casing so that it surrounds the axis L of the rotor 22, in order to drive a piston 42 of the cluster of axial piston cylinders A of an expander. Then a roller 73, arranged at the end of the piston 42 so that it can rotate freely, is brought into contact with the cam surface 25a. Thus the timing and the length of each process, such as an air intake process, an expansion process, an exhaust process or the like are set arbitrarily, and the efficiency of the expander can be increased, by driving the piston 42 at arbitrary timing and speed. Furthermore, since the roller 73 arranged at the piston 42 rolls on the cam surface 25a, the transfer of reaction force which is not conducive to the torque of the rotor 22 from the cam surface 25a to the piston 42 is minimized; and whirl on the sliding surface of the piston 42 and a cylinder sleeve 41 is prevented, and durability is increased.