At the time of external charging, an external charging relay is turned on, and a system main relay is turned off. Therefore, a main battery can be charged by electric power from an external power source, without applying an output voltage of the main battery to a PCU. At the time of external charging, basically, a power supply voltage for an auxiliary system (low voltage system) is generated by a sub DC/DC converter of a small capacity, while a main DC/DC converter of a large capacity used when vehicle is running is stopped. Only when the output of sub DC/DC converter is insufficient to cover the power consumption by the auxiliary system, an ECU operates the main DC/DC converter. Therefore, ECU and auxiliary load group can be operated reliably while efficiency of external charging is improved.