Deterioration of a selective catalytic reduction (SCR) catalyst can be diagnosed with sufficient accuracy, by making use of a sensor for measuring an air fuel ratio of exhaust gas flowing into an exhaust gas purification apparatus, which is equipped with the SCR catalyst, and a sensor for measuring an air fuel ratio of exhaust gas flowing out from the exhaust gas purification apparatus. In an exhaust gas purification apparatus for the exhaust gas purification apparatus, in a period of time in which inducement-processing is carried out which is to induce a water gas shift reaction in a catalyst disposed at the upstream side of the SCR catalyst, by changing an air fuel ratio of exhaust gas discharged from an internal combustion engine from a lean air fuel ratio into a predetermined rich air fuel ratio, an air fuel ratio of exhaust gas flowing into the SCR catalyst and an air fuel ratio of exhaust gas flowing out from the SCR catalyst are measured by air fuel ratio sensors, respectively, and deterioration of the SCR catalyst is diagnosed based on a difference between the measured values of these sensors, wherein the inducement-processing is carried out when the amount of NH3 adsorption in the SCR catalyst is equal to or less than a predetermined amount.