Diagnosis system of exhaust aftertreatment apparatus for internal combustion engine

A diagnosis system of an exhaust aftertreatment apparatus (17) for an internal combustion engine (1) is arranged to execute a first deterioration diagnosis (S2) of the exhaust aftertreatment apparatus on the basis of a change of a catalyst downstream side air/fuel ratio relative to a change of a catalyst upstream side air/fuel ratio during a rich spike control and to execute a second deterioration diagnosis (S5) from an inversion cycle of a feedback quantity during the feedback control of the catalyst downstream side air/fuel ratio during a stoichiometric control when the first deterioration diagnosis made a deterioration determination.