Intermittent short-circuit control is performed in the ignition mode, thereby allowing short-circuit current to flow in the current source step-down type chopper. Energy of the short-circuit current is temporarily accumulated in the inductor provided in the current source step-down type chopper. The accumulated energy boosts the output voltage from the direct current power supply device during the period until the next short circuiting, via the current, the multiphase inverter, and the rectifier. The voltage boosting operation where accumulation of the current energy by short circuiting and boosting of the output voltage by conduction are repeated, controls an increase of the output voltage which is applied to the plasma generator.