The output of an oxygen concentration detector monitoring the conditions of exhaust gases from an internal-combustion engine is not a linear function of the concentration but varies widely. When feeding this output back to control the air-fuel ratio of the mixture being fed to the engine, the feed back system having the small time constant (small delay time in operation) becomes unstable due to the nonlinear output of the oxygen concentration detector. In order to stabilize the operation of the system the output of the oxygen concentration detector is input to compensating circuit which outputs the voltage being proportioned to the oxygen concentration of the exhaust gas and the output is used as the feedback signal.