Disclosed herein is a comparator circuit used in a system for controlling, in accordance with an air-fuel ratio signal from an O.sub.2 sensor arranged in an exhaust system of an internal combustion engine, the air-fuel ratio of the exhaust gas. The comparator has two inputs, one of which receives a signal having a phase which is the same as a signal from the O.sub.2 sensor, the other of which receives a delayed phase signal. The output of the comparator operates to provide, in accordance with the voltage level difference between the inputs, two deviation signals, one of which indicates that air-fuel ratio has deviated to the rich side of the air-fuel ratio the other of which indicates that the air-fuel ratio has deviated to the lean side of the air-fuel ratio.