Abstract: A series circuit of two switching devices and a series circuit of two diodes are connected together in parallel to constitute a bi-directional switch circuit for one phase. N bi-directional switch circuits are provided, and the junction between the switching devices of each of the switch circuits is connected to an AC input terminal of a corresponding phase via a reactor. A cathode of the diode series circuit of each switch circuit is connected to a positive output terminal via a diode, and an anode of the diode series circuit of each switch circuit is connected to a negative output terminal via a diode. Two capacitors are connected in series between the positive and negative output terminals, and the junction between the diodes of each switch circuit is connected to the junction between the two capacitors.