PURPOSE: To immediately turn ON a semiconductor AC-SW, and to suppress the drop of an output waveform to a minimum limit by applying charge stored in a capacitor to the AC-SW as a differentiated waveform prior to a series of firing pulse train.

CONSTITUTION: When a malfunction occurs in an inverter so that an AC-SW ON signal is input, the switching means of a high speed AC-SW AMP 6 is turned ON, charge stored in a capacitor 7 is discharged through a primary coil, and a differentiated waveform B is supplied to a secondary coil. Since thyristors 3, 4 are turned ON by the waveform B, it is immediately switched to a commercial power source and supplied to a load. Accordingly, the power is supplied in a state that the drop of the output waveform is not substantially generated like a broken line near a symbol A. After a first firing pulse designated by the waveform B is supplied, a series of firing pulses C are supplied from the amplifier 5 to the gates of the thyristors 3, 4. Accordingly, the power supply thereafter is preferably conducted.