PURPOSE: To simplify circuit constitution and to increase power source efficiency, by a method wherein a dummy load circuit is installed between a rectifier circuit and an output terminal.

CONSTITUTION: The output from a DC power source 2 is regulated by a switching regulator comprising a transistor Q1, a transformer 1 and rectifiers 4 and 5, and then supplied to a load. The voltage comparator 11 compares detected output voltage with a set voltage, and controls duty ratio of the transistor Q1 through an oscillator 13 thereby controls the output voltage. Dummy load circuits 30 and 40 are connected between the rectifiers 4 and 5 and the output terminals. The power source voltage is smoothed by a diode 31 and a capacitor 32, and divided for application to the base of a transistor Q3. When the load is not connected, the power source voltage becomes high and the transistor Q3 is rendered on and current flows therethrough, so that duty of the transistor Q1 is not dropped excessively. Further, since a voltage detecting resistor is unnecessary, the circuit is simplified and the power source efficiency is good.