A switching power-supply circuit includes a second rectifying/smoothing circuit arranged to generate a second output voltage by rectifying and smoothing the output of a second secondary winding, and the second rectifying/smoothing circuit includes a second rectifier circuit and a capacitor, connected to the second secondary winding. A second switching control circuit operates in response to an alternating-current winding voltage occurring in the second secondary winding, and includes a time constant circuit causing a switch mechanism connected to the control terminal of a rectifier switch element to operate, and a second feedback circuit arranged to detect and feed back the second output voltage to the time constant circuit.