A power supply device includes a step-down unit to step down an input voltage, a switching unit to perform switching on a stepped-down voltage obtained through the stepping down by the step-down unit so as to externally output the voltage, an output variation detection unit to detect a corresponding variation of output from the switching unit, a delay unit to delay the input voltage by a prescribed time period, a delay variation detection unit to detect a corresponding variation of a delayed voltage output from the delay unit, an addition unit to add corresponding variations of the power supply voltage and the delayed voltage respectively detected by the output variation detection unit and the delay variation detection unit, and a control unit to perform feedback control on the basis of the corresponding variations of the power supply voltage and the delayed voltage added by the addition unit.