PURPOSE: To make a switching regulator of a pulse width controlling system appropriate to input power supply voltage dropping or hitting time in accordance with the weight of a load.

CONSTITUTION: A hit detection circuit 3 generates an input power supply abnormality signal when the level of input power supply drops for more than fixed time. When the width of a switching pulse abnormally increases, a pulse width abnormality detection circuit 10 generates a pulse width abnormality signal. An AND circuit 11 generates an abnormality signal only when both abnormality signals are simultaneously generated.