The present invention provides a power source device for supplying a load with power by combining a secondary battery and a capacitor, the power source device comprising: a bypass switch that is switched to a connection state so that the capacitor can supply the load with power directly from the capacitor when the voltage of the capacitor is high enough to drive the load; and a first DC-DC converter that boosts the voltage of the capacitor so as to supply the load with power when the voltage of the capacitor drops below a minimum voltage capable of driving the load.