Waste heat regeneration system

A waste heat regeneration system for a vehicle having a vehicle engine actuated by a start-stop switch includes Rankine cycle circuit, a motor generator, a by-pass circuit and a control device. The Rankine cycle circuit includes a pump, a boiler heating the heat medium by heat exchanging with waste heat generated by the vehicle engine, an expansion device and a condenser. The by-pass circuit is connected to the Rankine cycle circuit at the upstream and downstream sides of the condenser and the communication of the heat medium is openable and closable therethrough. When the start-stop switch of the vehicle engine is turned off, the control device controls the by-pass circuit to communicate the heat medium therethrough and keeps controlling of the rotational speed of the motor generator until pressure difference between the upstream and downstream of the expansion device is decreased to a predetermined level, and then stops the control.