PURPOSE: To reduce the size of a power conversion section, and improve the mounting density thereof, and further, decrease the number of the parts thereof.

CONSTITUTION: Power conversion sections INV 11, 12 are controlled respectively via FETs 13, 14 by switching over the output of an OSB 19 of a pulse width control section 1A through DRVs 24, 25. An EA 21 compares a sine-wave output 16 of an OSA 10 with the sensed single-phase AC output voltage of a constant- voltage.constant-frequency generator, and the taken-out difference between them is amplified to be outputted from the EA 21. When the output of the EA 21 is positive, a CNV 22 outputs a low voltage to the OSB 19, and performs the frequency modulation of the OSB 19. Similarly, when the output of the EA 21 is negative, a CNV 23 performs the frequency modulation of the OSB 19.