In discharging a smoothing capacitor connected to DC sides of first and second switching circuits, a DC-DC converter is first turned on to charge a battery for an auxiliary, thereby discharging the smoothing capacitor. Therefore, an electric charge of the smoothing capacitor can be effectively utilized to the maximum. When the voltage of the smoothing capacitor detected by a voltage detector is reduced to a predetermined value or less, and as a result the DC-DC converter is brought into an inoperable state, electric charge of the smoothing capacitor is consumed by at least one of a motor/generator and an LC filter connected to AC sides of the first and second switching circuits by driving at least one of the first and second switching circuits. Therefore, the smoothing capacitor can be completely discharged without need for a special discharging circuit.