To solve a problem wherein troubles occur where the output voltage becomes unstable, since the on-time period of a switching transistor becomes further shorter when the output voltage is switched to a voltage lower than a normal output voltage, and the control of the on-time period by a pulse width control circuit become unstable, when the on-time period of the switching transistor becomes too short, while in a conventional switching power supply, the on-time period of the switching transistor becomes shorter, as a load becomes lighter.

When an intermediate tap is installed on a primary-side coil of a high-frequency transformer and a normal output voltage is used for operation, power is supplied between one end of the primary-side coil and the intermediate tap, and in a state where the output voltage is switched to the lower voltage, power is supplied between the one end and the other end of the primary side coil.