A Rankine cycle system includes: an evaporator for heating a liquid-phase working medium with thermal energy of exhaust gas of an engine so as to generate a gas-phase working medium; a displacement type expander for converting the thermal energy of the gas-phase working medium generated by the evaporator into mechanical energy; a target pressure setter for setting a target pressure for the gas-phase working medium based on actual flow rate and temperature of the gas-phase working medium supplied to the expander; an estimated flow rate calculator for calculating an estimated flow rate of the gas-phase working medium supplied to the expander based on an output and a rotational speed of the engine; and a target rotational speed calculator for calculating a target rotational speed for the expander based on the estimated flow rate calculated by the estimated flow rate calculator and the target pressure set by the target pressure setter.