PURPOSE: To prevent generation of a wasteful power loss generated at a starting resistor by separating the resistor from a control IC after the IC is started.

CONSTITUTION: When a control IC 5 is started and a MOSFET 4 is controlled ON, OFF at a predetermined time ratio, a pulse current flows to a primary winding 3P, an auxiliary winding voltage is generated in an auxiliary winding 3A, and a capacitor 6C is charged through a diode 6D. The auxiliary winding voltage is applied to a gate of a triac 14, which is in a continuity, and a base B of a transistor 12 is lowered to a substantially ground potential. As a result, a current flowing to a starting resistor 10 is interrupted. Accordingly, thereafter, a power source voltage VCC is supplied from a power source 6 to the IC 5 to perform a normal operation, and power loss of a starting resistor 10 becomes zero. As a consequence, a power conversion efficiency of a switching regulator can be improved, for example, by about 1 to 2% as compared with that of prior art.