To provide a supply voltage drop detection circuit constituted of few circuit elements, capable of detecting either or both voltage drops of positive and negative voltage outputs of a DC/DC converter.

A voltage detection circuit acquires a voltage output corresponding to drop of the positive and negative voltage output in a resistance circuit 6 by using either of positive and negative voltage output terminals as a reference voltage, by connecting a pairs of resistant circuits 5, 6 and a series circuit of a Zener diode 7 to the positive and negative voltage output terminals of the DC/DC converter. A photocoupler 8 acquires existence of the drop of the positive and negative voltage outputs as an ON/OFF signal by using a voltage output of the voltage detection circuit as a comparison input and by using a threshold level carried by a photodiode D of the photocoupler as a comparison standard. The Zener diode includes a constitution wherein a plurality of Zener diodes are connected in parallel.