A deterioration monitoring apparatus for an exhaust system of an internal combustion engine includes an air-fuel ratio control system and a deterioration determining circuit. The air-fuel ratio control system controls an air-fuel ratio of exhaust emissions flowing downstream of a catalytic converter disposed in the exhaust system of the engine to agree with a target downstream air-fuel ratio under feedback control based on an air-fuel ratio of exhaust emissions flowing upstream of the catalytic converter and the air-fuel ratio of the exhaust emissions flowing downstream of the catalytic converter. The deterioration determining circuit determines whether the exhaust system such as an oxygen sensor mounted downstream of the catalytic converter or the catalytic converter is deteriorated or not based on the air-fuel ratio of the exhaust emissions flowing downstream of the catalytic converter and the target downstream air-fuel ratio.