PURPOSE: To make an exhaust emission control ratio keep in a ternary catalytic converter so better, by detecting capacity deterioration in an air-fuel ratio sensor, while subtracting a specified compensation value from the desired injection quantity for compensation in time of the capacity deterioration.

CONSTITUTION: In the case where air-fuel ratio control takes place with an electronic control circuit 20, a fundamental fuel injection quantity is processed through operation on the basis of a state of operation in an internal-combution engine. And, on the basis of an air-fuel ratio detecting signal out of an air-fuel ratio sensor detecting the air-fuel ratio, the fundamental injection quantity is compensated. In addition, variation time per unit voltage in time of the air-fuel ratio detecting signal being altered from rich to lean is detected, and when the variation time is longer than the setting value at normal time, the fuel injection quantity is compensated into decrement. Thus, the controlling air-fuel ratio is prevented from slipping to the rich side in time of capacity deterioration in the air-fuel ratio sensor so that an exhaust emission control ratio in a ternary catalytic converter is maintainable so favorably.