To control an output of a power generator to a demand output without depending on a throttle valve in a series hybrid vehicle provided with an electric motor as a prime mover and an engine driving the power generator as its power supply.

A supercharging compressor 5, a bypass passage 6 for the recirculation of supply air to an upstream side of the supercharging compressor 5 from between the compressor 5 and the throttle valve 2, and a bypass valve 7 opening and closing the passage are provided in a suction passage 1 of the engine. A means for detecting actual output of the power generator and a means for adjusting a degree of opening of the bypass valve 7 to match an output of the power generator with a demand output based on a detection signal are provided.