The deterioration judgment for an absorption reduction type NOx catalyst is performed quickly and correctly. A catalyst deterioration judging system includes a judging unit which judges that the absorption reduction type NOx catalyst is deteriorated if a detected value of an NH3 detecting unit is not more than a threshold value within a predetermined time immediately after starting supply of a reducing agent from a supply unit 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 when NOx is absorbed by the absorption reduction type NOx catalyst.