A Rankine cycle system is provided in which, with regard to a given relationship between the pressure (Pevp) and the temperature (Tevp) of a vapor that is taken into an expander (4) that includes a cylinder chamber in a first stage and a vane chamber in a second stage, the chambers being disposed in line, the expansion ratio of the vapor that the expander (4) takes in and discharges is set at a predetermined expansion ratio (ε) according to the given relationship so that the pressure (Pexp2) and the temperature (Texp2) of the vapor that is discharged from the expander (4) coincide with target values, thereby making the expander (4) and the condenser (5) exhibit maximum performance. Since the vapor within the cylinder chamber in the first stage is in a superheated vapor region and contains no water, the phenomenon of water hammer will not be caused in the cylinder chamber. Furthermore, since the vapor at the exit of the vane chamber is in a saturated vapor region and contains water, the vane chamber can be lubricated and sealed by means of the water.