To stabilize input voltage to a control circuit by improving the efficiency of the control circuit for activating a switching power supply.

In the switching power supply, a primary winding 11 of an output transformer is connected with a first rectifying/smoothing circuit A consisting of a rectifier 2 and a smoothing capacitor 3, a switching element 8 and a control circuit 6 for controlling such a switching element 8, the switching element 8 is connected to an auxiliary winding 13 of the output transformer, and the auxiliary winding 13 is connected with a second rectifying/ smoothing circuit B, consisting of a rectifying diode 9 and a smoothing capacitor 10. An input voltage to the control circuit 6 is divided with a resistor 4a and a constant voltage diode.