Discharge of electricity from a drive battery is started, and after a lapse of a predetermined time from the start of supply of electric power to a V2X device or power source apparatus, quick charging contactors are opened (S10-S16) and then failure diagnosis is executed (S18). If a failure is detected, a warning is initiated and restart of the electric power supply is inhibited (S20, S20, S32, S34). On the other hand, if no failure is detected, an open circuit voltage is detected, and the remaining capacity of the drive battery is calculated based on the open circuit voltage (S24-S26). The calculated remaining battery capacity is memorized (S28). Subsequently, the electric power supply from the drive battery to the V2X device or power source apparatus is restarted (S30).