PURPOSE: To effectively use the output of a retarder to reduce the fuel consumption of an engine, by utilizing resistance as a heat source for Rankine cycle to recover the output of a generator as a work.

CONSTITUTION: A load resistor 15 is disposed in a Rankine cycle heater 17 to further heat up working fluid which has been preheated by an exhaust pipe 21, to convert the fluid into high temperature gas. The working fluid is fed into a turbine 16 for working, and is led into a condenser 19 through a regenerator 18 to be converted into a liquid which is pressurized and discharged out by means of a feed pump 20. Further, first stage preheating is carried out by means of the regenerator 18, and exhaust gas is heated by the exhaust pipe and is fed into the heater 17 disposed therein with the load resistor 1. With the repetition of the above-mentioned cycle, the Rankine cycle turbine 16 recovers energy when a retarder is operated.