A power supply device has a first node to which an input voltage is input, a second node, a third node from which an output voltage is output, a first switching circuit electrically connecting the first node and the second node and a resonance circuit electrically connecting the second node and the third node. The power supply device converts the input voltage into the output voltage through an intermediate voltage of the second node. A first timing circuit is operable to turn on and turn off the first switching circuit. When the intermediate voltage becomes equal to or larger than a first predetermined voltage, the first timing circuit turns on the first switching circuit after a first predetermined time period.