PURPOSE: To provide a switching power supply apparatus which has an output smoothing choke coil and is used for lighting a halogen lamp, etc. and which has the high voltage precision of line regulation, etc., and, further, which is simple in construction and low in cost.

CONSTITUTION: After an AC input is rectified by a rectifier stack DM1 and smoothed by a capacitor C1, the smoothed output is chopped by a switching device Q1 and then smoothed by a choke coil L2 and the capacitor C2 and supplied to a load. At that time, the voltage between both terminals of the single winding choke coil L2 is detected by a detecting transformer T2 and a polarity component which is proportional to the output voltage is detected by a detector circuit 6 from the output of the detecting transformer T2 and compared with a predetermined voltage. A PWM circuit 7 generates a pulse corresponding to the signal from the detector circuit 6 and drives a switching device Q1 through an insulating transformer T1 to control the output voltage.