A power supply device includes a rectifying unit for rectifying an AC input voltage, a voltage conversion unit including a switching element and configured to receive a rectified input voltage from the rectifying unit and obtain an output voltage voltage-converted by a switching operation of the switching element, and a voltage comparison unit configured to generate a comparison signal between the output voltage and a set output voltage. The power supply device further includes an oscillation unit configured to output a control signal having an oscillation period and an oscillation-stop period as a control signal for controlling an ON/OFF of the switching element. The oscillation period and the oscillation-stop period of the control signal are controlled by the comparison signal outputted from the voltage comparison unit, and a pulse duty ratio of the control signal in the oscillation period is variably controlled based on the input voltage.