To provide a switching power supply having a backup function at low cost which is small, with few number of components and having power loss for high conversion efficiency.

The switching power supply comprises a first converter, a charging/discharging circuit, a secondary battery, and a secondary converter. The secondary winding of the conversion transformer of the first converter is connected to a rectifying/smoothing circuit and the charging/discharging circuit. The secondary battery is charged by the charging/discharging circuit. If no power is supplied to the first converter, the output of the secondary battery is supplied to the second converter composed of a switching element and the rectifying/smoothing circuit through the charging/discharging circuit.