PURPOSE: To lower a frequency of a PWM control signal during the outage of commercial power and reduce a loss in proportion to the frequency of the PWM control signal, by stopping a dividing circuit when a charging signal is generated, and operating the dividing circuit when a discharging signal is generated.

CONSTITUTION: In a charging/discharging circuit, a battery 5 is charged by DC power from a rectifier 2 when the power is fed from a commercial power supply 1, and the power is discharged from the battery 5 when the commercial power 1 is stopped. A charging/discharging discriminator 6 for judging whether a battery 5 is in a changing state or not and generating a charging/discharging signal is provided in the charging/discharging circuit. At the same time, a divider circuit 3 is provided between an oscillating circuit 32 and a carrier generating circuit 33 in a PWM control circuit 30. When a charging signal is generated, the divider circuit 35 is not operated, and when a discharging signal is generated, the divider circuit 35 is operated. Then, a loss in PWM inverter is reduced during the discharging of the battery 5.