A switching power supply has a function of improving a power factor, and outputs insulated DC. The switching power supply performs two kinds of controls for switching devices exclusive to each other: controlling of a switching device provided in a direction in which the discharging of a primary-side smoothing capacitor is prohibited at the time of suspension of commercially available AC power and near the zero cross of an input voltage; and causing a switching device provided between the output side of a third winding and the primary-side smoothing capacitor to control a charging current, thereby charging a capacitor in a boosted manner.