A vehicle power source device reliably turns on a high voltage battery even if the output voltage of a low-voltage ancillary battery drops at the start of driving the vehicle. A request to run on a high-voltage battery (to turn on a relay contact point) is made by means of the operation to start driving a vehicle. A low-voltage ancillary battery and an auxiliary battery supply power to a power source circuit. A relay control unit supplied with power from the power source circuit turns on the relay contact point and turns on the high-voltage battery. When the high-voltage battery is on, the auxiliary battery is charged by means of a step-down circuit. The auxiliary battery holds the charged power by means of a diode without the power being consumed by a group of low-voltage loads.