The deterioration judgment for an absorption reduction type NOx catalyst (1) is performed quickly and correctly. A catalyst deterioration judging system comprises a supply unit (5) which supplies a reducing agent, a measuring unit (8) which measures a NOx concentration in an exhaust gas at a position downstream from the catalyst (1), a control unit (10) which regulates an amount of the reducing agent so that an air-fuel ratio of the exhaust gas is a lean air-fuel ratio when the reducing agent is supplied from the supply unit (5), and a judging unit (10) which judges deterioration of the catalyst (1) on the basis of the NOx concentration measured by the measuring unit (8) when the amount of the reducing agent is regulated so that the air-fuel ratio of the exhaust gas is the lean air-fuel ratio when NOx is absorbed by the catalyst (1).