To prevent a turn-off of an AC power source from being detected as an overcurrent, and to quickly detect an overcurrent when a load has abrupt change.

An output voltage applied to a load 15 is divided by resistances R1 and R2 and a division voltage is supplied to a three-terminal regulator 16 to be compared with a reference voltage Ref. A feedback current FB flows according to comparison output. A resonance control part 10 controls an oscillation frequency according to the level of the feedback current FB to stabilize the output voltage. A current detection circuit 14 detects a current flowing on a primary side. When a current equal to or larger than a predetermined value flows to the primary side, the resonance control part 10 detects abnormality, and overcurrent protection operation is carried out. The resonance control part 10 is made to have a function of changing an overcurrent detection point associatively with a resonance frequency. An optimum overcurrent detection point can, thereby, be set according to load conditions.