PURPOSE: To prevent distortion of the output voltage waveform of an AC commercial power supply even upon occurrence of higher harmonic current by controlling a PWM inverter as an active filter when the commercial power supply is normal.

CONSTITUTION: When a commercial power supply 1 is normal, an AC circuit breaker 3 is turned ON to feed an AC power to a load 2 and the load current is detected by a load current detector 20. A compensation current detecting circuit 21 detects trouble current components, e.g. higher harmonic components of the load current, reactive current, and the like, and a first control command for controlling the secondary voltage of a transformer 7, through a current control circuit 2, to a PWM inverter 5 with a current having reverse phase is selected. Consequently, the trouble current is canceled by the PWM inverter and when a battery is charged, only a sinusoidal basic wave component can be obtained from the AC commercial power supply.