To detect an overcurrent by use of a constant load power or predetermined load power when a DC power supply voltage in a primary side of a switching power supply changes.

A current flowing into a switching element 2 is detected by a current detecting resistor 3 and this current is then inputted to an overcurrent detecting terminal of a switching control IC 12 via a resistor 4. A waveform generated in a primary side auxiliary coil 14 is clamped to an adequate waveform of a voltage in the negative side with a Zener diode 7, the waveform is applied to the overcurrent detecting terminal of the switching control IC 12 through a resistor 6, the waveform can be detected at an overcurrent detecting threshold level of the switching control IC 12 even when a pulse width is narrow using the directly clamped voltage, and the overcurrent detection can be set with constant load power or with a desired load power.