PURPOSE: To prevent current (leader current) from flowing before the resistance between the main terminals lowers sufficiently by having a switching element start ON operation before energy stored in a transformer is discharged when an output is overloaded or short-circuited.

CONSTITUTION: An overload protection circuit is provided to detect an output voltage and to delay the on-time of a switching element (MOSFET1) by interrupting the operation of a switching control transistor 5 for a predetermined time when a detected output voltage lowers to a setpoint or less. The overload protection circuit comprises a second auxiliary winding (B), diode 21, capacitor 22, No.1-No.3 resistors 23-25, and No.1 and No.2 transistors 26, 27, in addition to a transformer 2.