Pistons (41) are slidably received in a plurality of cylinders (39) disposed radially in a rotor (31), and a plurality of vanes (42) cooperating with the pistons (41) are disposed radially in the rotor (31), so that a vane chamber (54) is defined between a pair of the adjacent vanes (42). The radial movements of each of the pistons (41) and each of the vanes (42) are substantially stopped, so that the volumes of each of the cylinders (39) and each of the vane chambers (54) are not changed, for a period from the end of an exhaust stroke at which a gas-phase working medium is discharged from the cylinder (39) and the vane chamber (54) to the start of an intake stroke at which the supplying of the gas-phase working medium is started. Thus, it is possible to prevent the generation of a water hammer phenomenon due to a liquid-phase working medium confined in the cylinder (39) and the vane chamber (54).