To reduce loss, to make battery life longer, and to reduce the capacity of an electric double-layer capacitor (EDLC).

The EDLC 10 is connected in parallel via a step-up and step-down chopper 17 to a battery 8 connected to an AC motor 13 via an inverter 12. A current of the EDLC 10 or the battery 8 is detected and a DC current flowing from the battery 8 to the inverter 12 is detected. Whether it is power running or regeneration is determined from the code of this DC current, and a command value of the battery current is set to power-running side or regeneration side according to the determination result. The command value of the battery current is subtracted from the DC current to calculate a command value of a chopper current. Then, switching the step-up and step-down chopper 17 is controlled so as to make the chopper current reach the command value.