A grounding wire from a commercial power system is connected to a series-connection end of a series connection between two capacitors connected in series between a positive electrode and a negative electrode. A current detector monitors an output current of an inverter unit in which four switching elements and two diodes convert voltages at both ends of the series circuit of the capacitors at three levels. An operation control circuit controls a generation of a PWM signal to be applied to the four switching elements, to minimize a difference between a current value detected by the current detector and a target current value.