PURPOSE: To increase the speed of response of a feedback circuit as a whole by connecting an emitter-earthed type amplifier circuit to the output circuit of the transistor on the light receiving element side of a photocoupler.

CONSTITUTION: Resistors 5 and 6 of a detection circuit 40 detect the output voltage VO of a switching regulator and a shunt regulator 8 compares the detected voltage with a reference voltage and controls an electric current IF1 flowing through the light emitting element 6 of a photocoupler based on the error voltage obtained as a result of the comparison. The electric current IC1 flowing to the light receiving element 7 of the photocoupler also flows to a resistor 1 which becomes a collector load and generates a voltage VB. When the voltage VB reaches a bias voltage across the base and emitter of a PNP transistor 9, the transistor 9 is energized and generates a voltage VS both ends of a resistor 2. Based on the voltage VS, a control circuit 11 maintains the output voltage VO at a fixed level by controlling a switch element 12 by performing pulse width modulation or frequency modulation.