An AC/DC switching power supply that realizes a high power factor and has little voltage stress on a smoothing capacitor is provided. Also a switching power supply that has little voltage stress on a switch and is suitable for reduction in loss and miniaturization is provided. A switching power supply comprises a rectifying circuit for rectifying an AC voltage, a smoothing capacitor for smoothing an output of the rectifying circuit, a series switch circuit formed by a first switch and a second switch connected between two electrodes of the smoothing capacitor, a transformer for inducing a voltage to be an output at a secondary winding as the first switch is turned on/off and the second switch is turned on/off in a complementary manner with the first switch, and a series circuit formed by a primary winding of the transformer connected between a connection point of the first switch and the second switch and one terminal of the smoothing capacitor, and a resonance capacitor, wherein a first magnetic element connected between a positive electrode obtained by rectifying the AC voltage and an intermediate tap of the primary winding is provided, the resonance capacitor is connected to the connection point of the first switch and the second switch, and the primary winding is connected to the positive electrode of the smoothing capacitor.