A Rankine cycle system for recovering a heat energy of an exhaust gas is mounted on a hybrid vehicle including an internal combustion engine as a drive source for the vehicle and an electric generator motor. An output from the Rankine cycle system is input to a transmission and used to assist in a driving force from the internal combustion engine, or converted into an electric power and used to charge a battery. During acceleration and cruising of the vehicle, the heat energy of the exhaust gas is recovered by the Rankine cycle system, and during deceleration of the vehicle, a kinetic energy of a vehicle body is recovered as a regenerative electric power for the electric generator motor, thereby reducing the amount of fuel consumed in the internal combustion engine. Thus, in any operational state, vehicle energy efficiency is enhanced, thus reducing the fuel consumed.