PURPOSE: To obtain a DC voltage whose ripple is small by installing a smoothing reactor, to reduce a loss generated in a switching element for an inverter and to enhance the efficiency of a DC-DC converter as a whole.

CONSTITUTION: When a gate signal at a V-phase rises, an inverted signal is input to an AND circuit 27 by a NOT circuit 25, its AND condition is not established, and a gate signal at a U-phase falls. In the same manner, when a gate signal at a Y-phase rises, an AND condition in an AND circuit 28 is not established, and a gate signal at an X-phase falls. Thereby, a current which continues to flow in a smoothing reactor 6 during a commutation period cannot flow to an inverter 3, and the steady ON-state loss of a transistor SU or the like is reduced sharply.