A diagnostic apparatus for an exhaust gas purification apparatus of an internal combustion engine, the exhaust gas purification apparatus including: a selective catalytic reduction NOx catalyst which is provided in an exhaust passage of the internal combustion engine and which uses ammonia to reduce NOx in exhaust gas; a supplying unit which is provided on an upstream side of the selective catalytic reduction NOx catalyst and which supplies ammonia or an ammonia precursor into exhaust gas that flows into the selective catalytic reduction NOx catalyst; a control unit configured to control a supply amount of the ammonia or ammonia precursor that is supplied from the supplying unit; and a sensor which is provided on a downstream side of the selective catalytic reduction NOx catalyst, which detects NOx and ammonia in exhaust gas that flows out from the selective catalytic reduction NOx catalyst, and at which NOx and ammonia react with each other, the diagnostic apparatus for the exhaust gas purification apparatus comprising: a NOx concentration estimating unit configured to estimate a NOx concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst; an ammonia concentration estimating unit configured to estimate an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst; a calculating unit configured to calculate an amount of decline in output of the sensor caused by a reaction between NOx and ammonia at the sensor, based on the NOx concentration estimated by the NOx concentration estimating unit and the ammonia concentration estimated by the ammonia concentration estimating unit; a diagnosing unit configured to perform a diagnosis of the supplying unit based on an output value of the sensor; and a diagnosis control unit configured to control a diagnosis by the diagnosing unit based on the amount of decline in output of the sensor as calculated by the calculating unit. a selective catalytic reduction NOx catalyst which is provided in an exhaust passage of the internal combustion engine and which uses ammonia to reduce NOx in exhaust gas; a supplying unit which is provided on an upstream side of the selective catalytic reduction NOx catalyst and which supplies ammonia or an ammonia precursor into exhaust gas that flows into the selective catalytic reduction NOx catalyst; a control unit configured to control a supply amount of the ammonia or ammonia precursor that is supplied from the supplying unit; and a sensor which is provided on a downstream side of the selective catalytic reduction NOx catalyst, which detects NOx and ammonia in exhaust gas that flows out from the selective catalytic reduction NOx catalyst, and at which NOx and ammonia react with each other, the diagnostic apparatus for the exhaust gas purification apparatus comprising: a NOx concentration estimating unit configured to estimate a NOx concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst; an ammonia concentration estimating unit configured to estimate an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst; a calculating unit configured to calculate an amount of decline in output of the sensor caused by a reaction between NOx and ammonia at the sensor, based on the NOx concentration estimated by the NOx concentration estimating unit and the ammonia concentration estimated by the ammonia concentration estimating unit; a diagnosing unit configured to perform a diagnosis of the supplying unit based on an output value of the sensor; and a diagnosis control unit configured to control a diagnosis by the diagnosing unit based on the amount of decline in output of the sensor as calculated by the calculating unit. a NOx concentration estimating unit configured to estimate a NOx concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst; an ammonia concentration estimating unit configured to estimate an ammonia concentration in the exhaust gas that flows out from the selective catalytic reduction NOx catalyst; a calculating unit configured to calculate an amount of decline in output of the sensor caused by a reaction between NOx and ammonia at the sensor, based on the NOx concentration estimated by the NOx concentration estimating unit and the ammonia concentration estimated by the ammonia concentration estimating unit; a diagnosing unit configured to perform a diagnosis of the supplying unit based on an output value of the sensor; and a diagnosis control unit configured to control a diagnosis by the diagnosing unit based on the amount of decline in output of the sensor as calculated by the calculating unit.