To drive an electric motor of three phase 100 V using single phase 100 V power supply by full-wave rectification of single phase 100 V AC power supply, and generating AC voltage of three phase 100 V from DC power supply outputted from a full-wave-rectifying circuit.

Single phase AC power supply outputted from single phase AC power supply 21 of 100 V is full-wave rectified in a full-wave-rectifying circuit 22. And then it is supplied to an inverter 23. In a control unit 38, gate signals G11 and G16, gate signals G13 and G12, and gate signals G15 and G14 are turned on alternately, and thereby, transistors Q11 and Q16, transistors Q13 and Q12, and transistors Q15 and Q14 are conducted alternately. At this time, by varying cycles for turning on the gate signals G11 and G16, the gate signals G13 and G12, and the gate signals G15 and G14, an AC frequency of three phase 100 V supplied to the electric motor 24 can be varied.