In a semiconductor device for controlling switching power supply of this invention, having a switching element and switching operation control circuit, after receiving a current detection signal when switching is turned off, a fixed delay time is applied to the current detection signal by a delay circuit so that switching turn-on control by a transformer reset pulse signal obtained based on a signal from the tertiary windings of the transformer is not accepted within a blanking time corresponding to the delay time. Thus, the switching by the switching element is halted.