A power supply circuit comprises a converter transformer and a switching transistor. The input winding of the transformer, the collector and emitter of the transistor, and a collector-emitter current detecting resistor are connected in series, while the base of the transistor is connected through a starting resistor to a direct current input and is also connected to one end of the feedback winding of the transformer. A turning off capacitor and a diode are connected in series between both ends of the feedback winding, whereby the capacitor is charged on the occasion of non-conduction of the transistor. A switching element of two transistors as cascade connected is connected directly or through the above described resistor between one end of the capacitor and the emitter of the transistor. A variable direct current voltage obtained based on the voltage from the detecting winding of the transformer and a voltage developed across the above described resistor are applied to the control input of the switching element, so that the switching element is rendered conductive when the voltage developed across the resistor reaches a predetermined value determined by the variable direct current voltage, whereby the transistor is turned off by the voltage across the capacitor.