In an on-vehicle electric power source system, a power source control device is configured to supply an electric power from an electric power supply source to an actuator by means of a dual-circuit power supply line. The dual-circuit power supply line includes a bypass line and a charge-up line arranged in parallel with the bypass line. A first shut-off circuit is disposed in the bypass line. A booster circuit, an electricity storage device, and a second shut-off circuit are disposed in the charge-up line. The storage device is provided for storing an electric power boosted up by the booster circuit. Also provided is a control unit, which is configured to perform switching between an electric power supply through the bypass line and an electric power supply through the charge-up line by a changeover of a shutoff-circuit established/blocked state between the first and second shut-off circuits.