PURPOSE: To improve precision of deterioration decision of a catalyst through prevention of the occurrence of erroneous decision when deterioration of a catalyst is decided based on the locus length of the output of an air-fuel ratio sensor situated downstream from a catalyst.

CONSTITUTION: A downstream air-fuel ratio sensor 15 is arranged downstream from the catalyst converter 12 of an exhaust gas system. When the locus length of the output of the downstream air-fuel ratio is increased, it is decided that the catalyst is deteriorated. In such a state that an air-fuel ratio in a spot situated upper stream from the catalyst is oscillated more widely and longer to the rich side than a theoretical air-fuel ratio, even when the catalyst is deteriorated, the locus of the output of the downstream air-fuel ratio sensor 15 is shortened, whereby it is sometimes erroneously decided that a deteriorated catalyst is normal. It is decided by a control circuit 10 from a value of an air-fuel ratio correction amount (RSR), calculated based on the output of the downstream air-fuel ratio sensor, whether or not the air-fuel ratio of a spot situated upper stream from the catalyst is brought into a state to be widely oscillated to the rich side (a state wherein erroneous decision is apt to occur). When the air-fuel ratio is brought into such a state that erroneous decision is apt to occur, decision of deterioration of the catalyst is prohibited.