A power supply circuit improves the power factor when the load is light, and incorporates a starting circuit for reacting quickly to the changes in the output from the power supply circuit and a short-circuit detecting means for detecting the short-circuit of the feedback signal. The apparatus includes: an error amplifier; a comparator that monitors the output from the error amplifier and generates an offset regulating current ISO; a multiplier; a sensing current comparator that compares the output signal from the multiplier and the AC line current and generates a reset signal; a timer that directly monitors the inputted zero-cross signal; and a comparator for short-circuit detection, that facilitates reducing the exterior parts and components.