In a switching power supply device, a snubber circuit includes a snubber capacitor, a FET, and a snubber inductor. The snubber capacitor stores therein surge power due to a reverse recovery time of FETs. The FET discharges the surge power stored in the snubber capacitor. The snubber inductor is provided in a stage following the FET. The controller controls an actual on-duty at which the FET of the snubber circuit is turned on based on the surge power. The controller can set an on-duty lower limit indicating the lower limit of the actual on-duty. The on-duty lower limit is smaller than the actual on-duty set for the FET in a continuous current mode in which an electric current flowing through the snubber inductor continuously flows and larger than 0.