PURPOSE: To shorten the storage time and the like of an apparatus by applying a reverse bias according to the collector current of a main transistor, when a signal from a control circuit changes to OFF and said main transistor shifts from ON to OFF.

CONSTITUTION: A switching power supply is composed of an input filter circuit 1, a main switching circuit 2, an output filter circuit 3, an auxiliary power circuit 4 furnished with voltage-stabilizing means, a drive circuit 5, and a control circuit 20. Said control circuit 20 is formed of an oscillator circuit 6, a triangular wave generator circuit 7, a reference voltage generator circuit 8, an error amplifier circuit 9, and a comparator circuit 10. Thus, said comparator circuit 10 compares and mixes a triangular wave from the triangular wave generator circuit 7 with an output voltage from the error amplifier circuit 9 to convert the mixed output into ON-OFF waveform to transmit it to the drive circuit 5. Then, said drive circuit 5 applies a reverse bias to the main transistor at the time of shifting from ON to OFF.