To provide a smaller and more efficient switching power supply, the switching power supply includes: a high frequency transformer; a primary side switching circuit adapted to switch a current flowing through a primary winding of the high frequency transformer to thereby generate predetermined voltages in a secondary winding and an auxiliary winding of the high frequency transformer; a secondary side rectifier adapted to rectify a voltage generated in the secondary winding to supply the rectified voltage to a load; a charging circuit adapted to rectify a current flowing through the auxiliary winding and to switch the rectified current to thereby charge a secondary battery at a constant current; and a switch, provided between the secondary battery and the load, which supplies, when the voltage supplied to the load is interrupted, the voltage of the secondary battery to the load without going through the high frequency transformer.