A switching power supply unit generates a pulse signal having a duty factor based on the voltage associated with the difference between a reference voltage and the output voltage of the unit, and the voltage associated with the current flowing through a smoothing coil. Upon receipt of the pulse signal and a light-load determination signal, a delay-control unit of the power supply unit outputs as an instruction signal supplied to the switching circuit the pulse signal as it is when the light-load determination signal indicates that the load is not light, but otherwise outputs the pulse signal after delaying and widening the pulse width thereof. Thus, when the load is light, the switching power supply unit may hold its switching frequency substantially low and constant without rendering the frequency bursting discontinuously.