A power supply circuit includes a switching element provided with a control terminal for changing between an ON state and an OFF state, and an LC resonant circuit connected to the switching element and has a predetermined resonant frequency. A resistor connects to the control terminal of the switching element, and a feedback winding applies a feedback voltage derived from a high frequency signal across the LC resonant circuit to the control terminal through the resistor. A bias circuit connected to the feedback winding provides a bias voltage superimposed with the feedback voltage to the control terminal through the feedback winding and the resistor for alternately switching between the ON and Off states. The feedback voltage is delayed by a time defined by the resistor before being applied to the control terminal of the switching element.