PURPOSE: To accurately diagnose the variation of characteristics of an oxygen sensor on the upstream side, by mutually comparing the rich/lean variations of the characteristics in both oxygen sensors on the upstream and downstream sides, under a condition where an air-fuel ratio is controlled by utilizing the oxygen sensor on the downstream side of an exhaust purifying catalyst.

CONSTITUTION: A fuel injection valve 6 is arranged in an intake manifold 5, in an internal combustion engine 1, and a three-way catalyst 10 is arranged in an exhaust manifold 8. In this case, respective oxygen sensors 16, 17 for detecting oxygen concentration in exhaust gas are arranged upstream/downstream from the three-way catalyst 10 in the exhaust manifold 8. The fuel injection valve 6 is controlled by a control unit 12 so that the air-fuel ratio of air-fuel mixture may approach to a target value based on the detection value of the oxygen sensor 16 on the upstream side. When a diagnostic condition of the oxygen sensor 16 on the upstream side is judged, a fuel injection quantity is controlled based on the detection value of the oxygen sensor 17 on the downstream side. In this case, the variation characteristics of both upstream/ downstream oxygen sensors 16, 17 are compared with each other, and the variation characteristics of the oxygen sensor 16 at the upstream side is self-diagnosed.