The present invention is directed to sufficiently reduce the level of EMI which occurs from a switching FET in a DC/DC converter and minimize deterioration in efficiency of a power supply. A DC/DC converter includes a switching circuit for driving a switching FET for increasing or decreasing voltage, and a switching circuit for driving a ringing frequency changing circuit. One end of a capacitor is connected to a drain of the switching FET for increasing or decreasing voltage, and the other end of the capacitor is connected to a drain of an FET for the ringing frequency changing circuit. A source of the FET for the ringing frequency changing circuit is connected to GND, and a control circuit is provided which makes the ringing frequency changing circuit valid so that a ringing frequency becomes low only in a ringing frequency component exerting large influence on deterioration in EMI.