The main electric power source is connected to a lower-voltage side of a DC/DC converter by a first switch, and an electrical double-layer capacitor and a regulator circuit is connected to a higher-voltage side of the DC/DC converter. In addition, permanent magnets are disposed on a rotor so as to generate a magnetic field that is in a reverse direction to excitation by a field coil inside a claw-shaped magnetic pole portion of the rotor. A power supply control circuit controls opening and closing of the first switch so as to be in an open state when the engine is at rest, and so as to be set to a connected state after the regulator circuit is operated to make the alternator generate electric power to precharge the electrical double-layer capacitor after the engine is started.