PURPOSE: To simplify the configuration of a circuit by setting a stable period in which a potential is not varied between waves of a modified wave, and providing an outage duration in which current is still between forward current and reverse current generated in a driving circuit.

CONSTITUTION: A control circuit 5 is formed to conduct a pulse width modulation, and has a first signal circuit 51 for a triangular wave α and a second signal circuit 52 for a square wave signal β. The wave α to be output is output as many square waves by pulse-width modulating a carrier C made of the triangular wave to be output from a carrier oscillator 6 according to a modulation signal M to be output from a mixer 10 by a modulator 8. The signal βis output as a waveform generated by a V/f converter 7 as it is to a signal divider 2a, but since a stable period exists between a positive potential period and a negative potential period of a AC regulated wave, the modulated wave is simultaneously varied at a peak and a width and not varied at a waveform, and hence an efficient correction is performed. As a result, a circuit configuration can be simplified.