A diagnostic apparatus for an exhaust gas purification apparatus including a selective catalytic reduction NOx catalyst (SCR catalyst) provided in an exhaust passage of an internal combustion engine, a supplying unit supplying ammonia or an ammonia precursor into exhaust gas flowing into the SCR catalyst and a sensor, provided on a downstream side of the SCR catalyst, detecting NOx and ammonia in exhaust gas. The diagnostic apparatus calculates an amount of decline in output of the sensor due to a reaction between NOx and ammonia at the sensor and performs a diagnosis of the supplying unit based on the calculated amount of decline in output of the sensor.