When a detection value of a temperature sensor exceeds a set temperature, an ECU increases an opening of a sub-port by controlling a solenoid valve, and as a result, an intake volume of an expander increases. The ECU then adjusts the mass flow rate of a coolant flowing through a boiler so that a detection value of a pressure sensor indicates a pressure as close as possible to an upper limit pressure, on condition that an upper limit temperature is not exceeded.