This detective circuit detects an abnormal drop of an input voltage of a switching regulator of the flyback system. From the secondary winding of the switching regulator, a constant voltage controlled dc voltage is delivered through a diode. This circuit is provided with a peak voltage charging circuit which detects the peak value of a voltage produced in a direction opposite to the forward direction of the diode, and a voltage comparator which compares the peak value with a setting value. Since this peak value is in proportion to the input voltage, the output of the voltage comparator indicates an abnormal drop of the input voltage.