A switching power supply achieves simultaneously achieves suppression of surge voltage and satisfactory transient response. The switching power supply comprises a switching element connected to a primary winding of an isolation transformer, a resonance capacitor connected to the switching element, an output capacitor connected to a secondary winding of the isolation transformer through a rectifying diode, and a control circuit ON/OFF driving the switching element. The control circuit includes a restart circuit that delivers a restart signal to turn ON the switching element after a predetermined time from turning OFF of the switching element has passed. The control circuit also includes an initial operation setting circuit that sets a delay time in the restart circuit to be longer in a start up period of the power supply than in a normal operation time after the end of the startup period.