PURPOSE: To prevent excess current from being generated, by providing the input of the reversed polarity of a actual voltage value to a voltage regulating means, when power source side voltage is lowered to come to a specified value or less, with a voltage controlling loop due to proportional integration arithmetic units.

CONSTITUTION: An inverter unit is composed of a rectifier 3, a smoothing capacitor 4, and a transistor inverter 5, and the AC power of desired voltage and frequency is fed to a load 6. To proportional integration arithmetic units 13A∼13C, the input of a deviation between an actual voltage value from a potential transformer 11 and a desired voltage value from a voltage setting unit 12 is provided, and a voltage regulator 13 is composed, and control signal is applied to the inverter 5 via a base driving circuit 14. In this case, a signal switching circuit 22 and a polarity reversing circuit 23 are arranged. In this state, when it is detected that an AC power source 2 is of specified voltage or lower, then the desired voltage value from the voltage setting unit 12 is interrupted, and the input of the actual voltage value of reversed polarity from the polarity reversing circuit 23 is provided, and overvoltage at the time of voltage recovery can be avoided.