To obtain a stable voltage even in the case of any rotation of an engine by incorporating a voltage dropping function in converting means for converting the output of rectifying means to three-phase AC power by a PWM control type.

A permanent magnet electric power generator 1 is driven by the engine of a vehicle, the generated output power of the generator 1 is stepped up by a rectifying booster 8, and the step-up output is rectified. The rectified output is supplied to an inverter 6 controlled by an inverter control circuit 7 of PWM control type. When the voltage is a target voltage or higher, the output power is turned on or off to step down the effective voltage. Thus, the step-down element of the output voltage of the generator 1 and the control circuit can be reduced, and stable voltage can be obtained by any engine rotation.