PURPOSE: To maintain an air-fuel ratio to a target value so as to improve exhaust gas purifying property by changing air-fuel ratio correction on the basis of an air-fuel ratio detecting means arranged downstream according to complex deterioration of the air-fuel ratio detecting means arranged upstream from an exhaust emission control catalyst in an internal combustion engine.

CONSTITUTION: In the intake passage 12 of an internal combustion engine 11, a fuel injection valve 15 is arranged downstream from a throttle valve 14. In an exhaust passage 18, first and second air-fuel ratio sensors 19, 21 are arranged above and below a catalytic converter rhodium 20. The fuel injection valve 15 is controlled by a control unit 16 on the basis of respective detection signals of at least respective air-fuel ratio sensors 19, 21. As the result, the air-fuel ratio of the internal combustion engine 11 is controlled. In this case, in the control unit 16, deterioration of the rich side output and responsiveness of the first air-fuel ratio sensor 19 are diagnosed on the basis of a first air-fuel ratio correction amount. As the result, when the deteriorations is diagnosed the second air-fuel ratio correction amount of the second air-fuel ratio sensor 21 is corrected in the direction to suppress rich control of the air-fuel ratio.