A deterioration diagnosis apparatus for an exhaust gas purification apparatus, wherein the exhaust gas purification apparatus is equipped with a selective catalytic reduction catalyst arranged in an exhaust passage of an internal combustion engine, which can be driven in a lean burn operation, for reducing NOx in exhaust gas by using ammonia as a reducing agent, said selective catalytic reduction catalyst being configured to store oxygen in exhaust gas when the air fuel ratio of the exhaust gas is a lean air fuel ratio higher than a stoichiometric air fuel ratio, but to desorb the oxygen thus stored when the fuel ratio of the exhaust gas is a rich air fuel ratio lower than the stoichiometric air fuel ratio, and an obtaining unit is configured to obtain an amount of NH 3 adsorption which is an amount of ammonia stored in said selective catalytic reduction catalyst; the deterioration diagnosis apparatus comprising: a first air fuel ratio sensor configured to measure a physical quantity correlated with an air fuel ratio of the exhaust gas flowing into said selective catalytic reduction catalyst; a second air fuel ratio sensor configured to measure a physical quantity correlated with an air fuel ratio of the exhaust gas flowing out from said selective catalytic reduction catalyst; a control unit configured to carry out inducement-processing which changes an air fuel ratio of a mixture to be combusted in the internal combustion engine from a lean air fuel ratio into a rich air fuel ratio, when the air fuel ratio of the mixture to be combusted in the internal combustion engine is a lean air fuel ratio and when the amount of NH 3 adsorption obtained by said obtaining unit is equal to or less than a predetermined upper limit value, said inducement-processing induces a water gas shift reaction of carbon monoxide and water generated at the time when the mixture is combusted in the internal combustion engine; and a diagnosis unit configured to carry out diagnosis processing for diagnosing deterioration of said selective catalytic reduction catalyst based on a difference between a measured value of said first air fuel ratio sensor and a measured value of said second air fuel ratio sensor at the time of execution of said inducement-processing. a first air fuel ratio sensor configured to measure a physical quantity correlated with an air fuel ratio of the exhaust gas flowing into said selective catalytic reduction catalyst; a second air fuel ratio sensor configured to measure a physical quantity correlated with an air fuel ratio of the exhaust gas flowing out from said selective catalytic reduction catalyst; a control unit configured to carry out inducement-processing which changes an air fuel ratio of a mixture to be combusted in the internal combustion engine from a lean air fuel ratio into a rich air fuel ratio, when the air fuel ratio of the mixture to be combusted in the internal combustion engine is a lean air fuel ratio and when the amount of NH 3 adsorption obtained by said obtaining unit is equal to or less than a predetermined upper limit value, said inducement-processing induces a water gas shift reaction of carbon monoxide and water generated at the time when the mixture is combusted in the internal combustion engine; and a diagnosis unit configured to carry out diagnosis processing for diagnosing deterioration of said selective catalytic reduction catalyst based on a difference between a measured value of said first air fuel ratio sensor and a measured value of said second air fuel ratio sensor at the time of execution of said inducement-processing.