Abstract: A hybrid vehicle driving system has a transmission means (22) for selectively transmitting the output torque of an internal-combustion engine (10), the output torque of an electric motor (19) or a composite torque produced by combining the respective output torques of the internal-combustion engine (10) and the electric motor (19) to driving wheels. The hybrid vehicle driving system includes a turbogenerator (7) capable of converting energy of the exhaust gas discharged from the internal-combustion engine (10) into electric energy, and an electrical connecting means (1) for electrically connecting the turbogenerator (7) to the electric motor (19). The hybrid vehicle driving system is controlled so that the engine (10) operates on a lean mixture for the largest possible part of the total operating time of the engine (10).