Waste heat recovery system for a vehicle

When a detection value of temperature sensor 15 exceeds set temperature T 1 , ECU 20 increases an opening of sub-port 24 by controlling solenoid valve 25, and as a result, an intake volume of expander 4 increases. The ECU 20 then adjusts the mass flow rate of a coolant flowing through boiler 3 so that a detection value of pressure sensor 16 indicates a pressure as close as possible to upper limit pressure Po, on condition that upper limit temperature T 0 is not exceeded.