A motor generator is connected to a high voltage battery through an inverter. A voltage of the high voltage battery is decreased and is applied to a low voltage battery through a DC-DC converter. At time of starting an engine, a starting operation of a starter is executed by applying an output voltage of the DC-DC converter 36 to the starter. At this time, the output voltage of the DC-DC converter 36 may be adjusted based on the temperature of the engine, the voltage of the low voltage battery and/or the degree of degradation of lubricant oil of the engine.