PURPOSE: To reduce the number of monitor pins of a differential amplifying circuit with monitor terminals.

CONSTITUTION: The input stage of a current mirror 40a constituting a reference level conversion circuit 40 is inserted and connected to a differential amplifying circuit 10. Then, operation signals of signals A and B are converted to a voltage signal C with ground GND as the reference by a resistance 40b through current signals F and B which are not affected by the variance of the supply voltage. Thus, the voltage signal C is checked through a pin for ground GND and a monitor pin 2, and the number of monitor pins which is two in a conventional circuit is reduced to one.