To provide a switching power supply capable of reducing a power loss in a standby mode.

The switching power supply comprises a transformer 1 having a primary winding, a secondary winding and an auxiliary winding, a semiconductor switch 2 connected in series with the primary winding, a control circuit 3 for turning the semiconductor switch 2 on and off periodically, a secondary winding rectifier circuit 5 connected with the secondary winding, an auxiliary winding rectifier circuit 25 connected with the auxiliary winding, a circuit 20 for detecting an output voltage from the secondary winding rectifier circuit, and a feedback circuit 30 for transmitting a detection signal from the output voltage detecting circuit 20 to the control circuit 3. When the output voltage from the auxiliary winding rectifier circuit 25 is not higher than a set level, a start compensation circuit 40 performs on/off-operation periodically even if there is no detection signal.