A power supply device includes a first power-supply unit to input external power and to output first power supplying a predetermined constant voltage to a load, a second power-supply unit to accumulate charge and to output second power to the load in parallel with the first power being supplied from the first power-supply unit, and a controller to output a control signal to prevent the second power-supply unit from supplying power in case of a current increase due to a dynamic load change, and to cause the first power-supply unit to supply power for the dynamic current increase. The first power-supply unit supplies power having a current value smaller than a predetermined maximum current of the first power-supply unit. The second power-supply unit supplies power for a shortfall current exceeding the predetermined maximum current of the first power-supply unit.