A power supply device having a transformer and a control circuit for controlling a power input to the transformer in accordance with a set value of an output voltage is provided. The power supply device includes an auxiliary winding provided in the transformer and a control power supply circuit configured to rectify AC power produced in the auxiliary winding to output a control voltage for operating the control circuit. The control power supply circuit includes a smoothing capacitor which is connected between output terminals for the control voltage and is to be charged with power obtained by rectifying the AC power, and a charge thinning circuit configured to intermittently charge the smoothing capacitor so that a voltage value between both ends of the smoothing capacitor reaches a set value of the control voltage.