A flyback type switching power supply includes between P and N of a direct current output a sudden load change detector circuit, which normally has no power consumption, that detects only a transient fluctuation of a direct current output voltage, and starts the switching of a primary side semiconductor switch when there is no load or a light load, even when the semiconductor switch is in an off state, thereby enabling the detection of the direct current output voltage in a tertiary winding, and suppressing a drop in the direct current output voltage.