CATALYST DETERIORATION DETERMINING SYSTEM

The deterioration judgment for an absorption reduction type NOx catalyst (4) is performed quickly and correctly. A catalyst deterioration judging system comprises a judging unit (10) which judges that the absorption reduction type NOx catalyst (4) is deteriorated if a detected value of an NH 3 detecting unit (8) is not more than a threshold value within a predetermined time immediately after starting supply of a reducing agent from a supply unit (5) while regulating an amount of the reducing agent so that the air-fuel ratio of an exhaust gas is a rich air-fuel ratio by means of a control unit (10) when NOx is absorbed by the absorption reduction type NOx catalyst (4).