Waste heat recovery system

A waste heat recovery system is for use with a power unit that includes an internal combustion engine (5). The waste heat recovery system includes a Rankine cycle device in which working fluid circulates through a pump (20A), a boiler (23), an expander (25) and then through a condenser (27), heat exchange occurs in the boiler between the working fluid and intake fluid (14, 15) that is introduced into the internal combustion engine while being cooled, a determination device for determining required cooling load for the intake fluid, a pressure reducing device (35) for reducing evaporation pressure in the Rankine cycle device, and a controller (11A) for controlling the pressure reducing device so as to reduce the evaporation pressure below a predetermined evaporation pressure if the required cooling load determined by the determination device exceeds a threshold.