PURPOSE: To reduce power consumption by cutting off after a riseup of output voltage a circuit which supplies a current to a control circuit when an input power source is connected.

CONSTITUTION: A switching power source has a transformer 6 performing voltage conversion, main switch 5 and an output rectifier circuit 8, while the switching power source is provided with a control circuit holding output voltage from an output terminal t2 to the constant voltage and a protecting circuit 2 for overvoltage, low voltage, etc. This circuit 2 connects a resistor 1, and the main switch 5 receives a signal from the circuit 2, driving the transformer 6. And the transformer 6 adds a winding N3, and is provided with a rectifying diode 7 and a pass capacitor 3 as the means supplying an operating power source to the circuit 2. Further for controlling this power source supply so as to be released, a transistor 9, circuit 10 for cutting off this transistor 9, and resistors 11, 12 are provided. Thus supply of a current to the capacitor 3 is started simultaneously with starting the circuit 2, and the transistor 9 is stopped by a riseup of output voltage.