A diagnosis system of an exhaust aftertreatment apparatus for an internal combustion engine is arranged to execute a first deterioration diagnosis 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 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.