A power supply device includes a DC-DC converter configured to regulate an output voltage based on an externally input instruction, and a discharge circuit connected to an output capacitor of the DC-DC converter, wherein the discharge circuit includes a switch element configured to switch between a conductive state and a nonconductive state according to an instruction, and a capacitor-resistor (C-R) circuit configured to charge electric charge accumulated in the output capacitor of the DC-DC converter when the switch element is conductive and to discharge the electric charge to ground when the switch element is nonconductive.