PURPOSE: To reduce the output capacity of a DC-DC converter, by adding the output voltage of the DC-DC converter to the output of a battery, and applying the added DC voltage to the DC input side of a power converter.

CONSTITUTION: A transistor 20B and a transformer 20C receive a DC signal from a battery 22. The signal is intermittently cut and converted into an AC signal. The AC signal voltage is increased. The output signal from the transformer 20C is converted into a DC signal through diodes 20D and 20E. The signal is smoothed through a capacitor 20G. Thereafter, the signal is supplied to the DC input side of an inverter 16A. The DC voltage across the capacitor 20G is added to the battery 22. Therefore, the total voltage of the DC voltage of the capacitor 20G and the output voltage of the battery 22 are applied to the DC input side of the inverter 16.