An apparatus for determining a deterioration of a condition of a three-way catalyst arranged in an exhaust passage of an internal combustion engine, comprising an upstream O.sub.2 sensor arranged in the exhaust passage upstream of the catalyst and a downstream O.sub.2 sensor arranged in the exhaust passage downstream of the catalyst. An air-fuel ratio feedback correction amount is calculated in accordance with the output of the upstream O.sub.2 sensor, and the rate of change of the air-fuel ratio feedback correction amount is decreased when it is determined that the condition of the catalyst has deteriorated.
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