Abstract: An electric power source device is used in a vehicle equipped with a generator and a battery. The device includes a current detector for detecting a first current flowing in the battery, a DC-DC converter, a capacitor connected to the generator via the DC-DC converter, and a controller. The controller determines the state of charge of the battery based on a first input current detected by the current detector, and detects the running condition of the vehicle. When the vehicle decelerates, the controller controls, based on the determined state of charge of the battery, a second input current input to the capacitor via the DC-DC converter. Thereby, a regenerative electric power generated by the generator is used during the deceleration of the vehicle according to the SOC, thus utilizing the energy that is not conventionally.