A diode rectifier rectifies an AC voltage output from a neutral-grounding three-phase AC power supply to a DC voltage. A smoothing capacitor smoothes the DC voltage rectified by the diode rectifier. An inverter converts the DC voltage smoothed by the smoothing capacitor into a desired AC voltage. A bidirectional switch and a Y capacitor are connected in series between one end of the smoothing capacitor and the ground. A detector detects a voltage across both ends of the smoothing capacitor. A controller drives the bidirectional switch in response to an output of the detector.