PURPOSE: To simplify control of an inverter by providing functions of inputting a pulse corresponding to a commercial power source frequency and a pulse corresponding to an output frequency of the inverter and varying a period of a clock pulse to be sent based on its phase difference and a frequency deviation in a microcomputer.

CONSTITUTION: A pulse Pc corresponding to a frequency of a commercial power source 10 from a converter 5 and a pulse Pi corresponding to an output frequency of an inverter 3 from a converter 6 are input to a microcomputer 7 to detect a phase difference and a frequency deviation. A period of a clock pulse to be sent from a clock pulse generator is varied based on the phase difference and the frequency deviation, input to the inverter 3 through an output frequency setter 11, and synchronized with the power source 10. Here, corresponding values of instantaneous values of the output sine wave are stored in the number of the clock pulses during one period in a ROM, and the corresponding values are output from the ROM at each time of sending the pulse to the inverter 3 through the setter 11.