A switching power supply system has a control circuit that controls an output voltage by causing a switching device to turn ON and OFF. The control circuit includes a control pulse supplying unit that supplies a pulsed signal that keeps the switching device turned-ON and -OFF. A protection circuit shuts down the switching power supply system upon occurrence of an abnormality. A delay circuit produces a delay signal that delays by a specified time duration the termination of a state of the pulsed signal in which the pulsed signal keeps the switching device turned-ON. The protection circuit is responsive to the pulsed signal or the delay signal to switch between an operation state and a stand-by state.