PURPOSE: To operate a generator only upon necessity and improve the fuel consumption thereof by a method wherein a prime mover and the generator are coupled by a coupler and the generator is operated or stopped by a control unit through the coupler in accordance with the opening degree of a throttle valve.

CONSTITUTION: The coupler 3 is in a coupled condition at all times when there is no current input and the rotation of the prime mover 1 is transmitted to an auxiliary generator 2. When the opening degree of the throttle valve is in a specified level, the output of the signal of a throttle opening degree detector 4 is low and a relational controller 5 will never output a control current to the coupler 3. Accordingly, the generator 2 continues to generate a power by the rotation of the prime mover 1. On the other hand, when the opening degree of the throttle valve is increased for running a vehicle, the output of said detector 4 is increased, and when it is arrived at a predetermined value, the relational controller 5 is operated to output the control current to the coupler 3. As a result, the coupler 3 separates the generator 2 from the prime mover 1 and, accordingly, the output of the prime mover 1 is totally transmitted to a driving system and is consumed effectively for the running only.