To provide a switching power supply that enables simplification and making a DC voltage outputting circuit compact for outputting a stable DC voltage to the secondary side.

This power supply is provided with a transistor Q1 that switches on or off a second rectifier circuit 13 and a second smoothing circuit 15, a base current control circuit 20 that controls the base current, and an output voltage detecting circuit 30 that detects a second output voltage. The transistor Q1 is turned on or off based on the timing of rectified pulse waveforms from the second rectifier circuit 13. The base current control circuit 20 controls the base current at the 'on' time of the transistor Q1, so as to make the second output voltage constant based on the detected voltage value outputted from the output voltage detecting circuit 30.