PURPOSE: To inhibit the fluctuation of output voltage from an inverter main circuit by bringing a frequency ratio between a sinusoidal signal and a triangular wave signal to an integer at all times regardless of the selection of frequency.

CONSTITUTION: A triangular-wave generating circuit 11 is constituted of a phase-locked-loop circuit, and generates a triangular wave signal having frequency of integer times as many as a sinusoidal signal. The frequency-dividing ratio of the phase-locked-loop circuit is controlled by a frequency-dividing control circuit 17, and the frequency of the triangular wave signal is controlled within a predetermined range so that a pulse-width modulation signal sufficiently approximate to a sine wave is acquired and the repetition of ON-OFF commands based on the pulse-width modulation signal does not exceed the ON-OFF working speed of a switching element for an inverter main circuit.