An auxiliary device system battery and a small electric power load are connected to a first power supply line, and power supply power stepped up by a DC/DC converter in a zone ECU is supplied to a second power supply line. A large electric power load and a capacitor are connected to the second power supply line, and when the large electric power load is driven, the capacitor is connected to supply necessary power supply power, and voltage fluctuation is prevented. When an ignition is OFF, a switch circuit is closed, a dark current is supplied from the capacitor, and electric charge is discharged to prevent deterioration. Alternatively, the auxiliary device system battery is connected to a second power supply line side, and a dark current is supplied to a first power supply line side when the ignition is OFF using a capacitor or a dedicated circuit.