A power supply device for supplying power to a load by combining a secondary battery and a capacitor , comprising: a first DC-DC converter that is connected between the capacitor and the load and adapted to enable a voltage of the capacitor to be stepped up and supplied to the load ; a second DC-DC converter that is connected between the secondary battery and the load and adapted to enable a voltage of the secondary battery to be stepped up and supplied to the load or the capacitor ; characterised by a bypass switch adapted to enable power to be directly supplied to the load from the capacitor by bypassing the first DC-DC converter when being switched to a connected state; and a controller adapted to control the first DC-DC converter and a connecting state of the bypass switch , said controller being further adapted for: switching the bypass switch to the connected state to directly supply power to the load from the capacitor when a voltage of the capacitor is equal to or higher than a first set voltage that is a minimum voltage capable of driving the load ; switching the bypass switch to the shut-off state and controlling the first DC-DC converter to step up the voltage of the capacitor to supply power to the load when the voltage of the capacitor drops below the first set voltage; and controlling the second DC-DC converter to step up the voltage of the secondary battery to supply power to the load when the voltage of the capacitor drops below a second voltage that is a voltage higher than a minimum voltage at which the capacitor is operable, by a margin voltage. a first DC-DC converter that is connected between the capacitor and the load and adapted to enable a voltage of the capacitor to be stepped up and supplied to the load ; a second DC-DC converter that is connected between the secondary battery and the load and adapted to enable a voltage of the secondary battery to be stepped up and supplied to the load or the capacitor ; characterised by a bypass switch adapted to enable power to be directly supplied to the load from the capacitor by bypassing the first DC-DC converter when being switched to a connected state; and a controller adapted to control the first DC-DC converter and a connecting state of the bypass switch , said controller being further adapted for: switching the bypass switch to the connected state to directly supply power to the load from the capacitor when a voltage of the capacitor is equal to or higher than a first set voltage that is a minimum voltage capable of driving the load ; switching the bypass switch to the shut-off state and controlling the first DC-DC converter to step up the voltage of the capacitor to supply power to the load when the voltage of the capacitor drops below the first set voltage; and controlling the second DC-DC converter to step up the voltage of the secondary battery to supply power to the load when the voltage of the capacitor drops below a second voltage that is a voltage higher than a minimum voltage at which the capacitor is operable, by a margin voltage. switching the bypass switch to the connected state to directly supply power to the load from the capacitor when a voltage of the capacitor is equal to or higher than a first set voltage that is a minimum voltage capable of driving the load ; switching the bypass switch to the shut-off state and controlling the first DC-DC converter to step up the voltage of the capacitor to supply power to the load when the voltage of the capacitor drops below the first set voltage; and controlling the second DC-DC converter to step up the voltage of the secondary battery to supply power to the load when the voltage of the capacitor drops below a second voltage that is a voltage higher than a minimum voltage at which the capacitor is operable, by a margin voltage. <EOS>