PURPOSE: To prevent decrease in diagnostic accuracy caused by deviation of a rear air-fuel ratio and deviation of a judging standard by inhibiting deterioration diagnosis of catalyst in the case renewal of learning compensation which uses a downstream air-fuel ratio sensor is not performed enough.

CONSTITUTION: A catalyst converter 27 is interposed at an exhaust passage 23 in an internal combustion engine 21, and an upstream oxygen sensor 28 is arranged above the catalyst converter 27, and a downstream oxygen sensor 29 is arranged downstream. A feedback compensation coefficient is calculated based on output of the oxygen sensor 28 in a control unit 32, and the feedback compensation coefficient is corrected by a learning value based on the output of the oxygen sensor 29. When the number of renewal of the learning value based on the output of the oxygen sensor 29 is at a specified value or more, the deviation in the air-fuel ratios is seldom happened, and deterioration diagnosis of the catalyst converter 27 is performed by comparing the outputs of the oxygen sensors 28, 29 each other. Consequently, the deterioration diagnosis is performed accurately.