An abnormality diagnosis device for a sensor in an exhaust gas purification apparatus of an internal combustion engine, which is provided with: an NOx selective reduction catalyst that is arranged in an exhaust passage of the internal combustion engine and uses ammonia as a reducing agent; a supply unit configured to supply ammonia or a precursor of ammonia into an exhaust gas flowing into said NOx selective reduction catalyst, at a location upstream of said NOx selective reduction catalyst; and a sensor that is arranged at the downstream side of said NOx selective reduction catalyst, and configured to detect NOx and ammonia in an exhaust gas flowing out of said NOx selective reduction catalyst, with the NOx and the ammonia reacting with each other therein; comprising: an NOx concentration estimation unit configured to estimate a concentration of NOx in the exhaust gas flowing out of said NOx selective reduction catalyst; an ammonia concentration estimation unit configured to estimate a concentration of ammonia in the exhaust gas flowing out of said NOx selective reduction catalyst; a calculation unit configured to calculate an amount of decrease in an output of said sensor due to a reaction of NOx and ammonia in said sensor, based on the NOx concentration estimated by said NOx concentration estimation unit and the ammonia concentration estimated by said ammonia concentration estimation unit; a diagnostic unit configured to carry out a diagnosis of said sensor based on an output value of said sensor; and a diagnostic control unit configured to control the diagnosis in said diagnostic unit based on the amount of decrease in the output of said sensor calculated by said calculation unit. an NOx selective reduction catalyst that is arranged in an exhaust passage of the internal combustion engine and uses ammonia as a reducing agent; a supply unit configured to supply ammonia or a precursor of ammonia into an exhaust gas flowing into said NOx selective reduction catalyst, at a location upstream of said NOx selective reduction catalyst; and a sensor that is arranged at the downstream side of said NOx selective reduction catalyst, and configured to detect NOx and ammonia in an exhaust gas flowing out of said NOx selective reduction catalyst, with the NOx and the ammonia reacting with each other therein; comprising: an NOx concentration estimation unit configured to estimate a concentration of NOx in the exhaust gas flowing out of said NOx selective reduction catalyst; an ammonia concentration estimation unit configured to estimate a concentration of ammonia in the exhaust gas flowing out of said NOx selective reduction catalyst; a calculation unit configured to calculate an amount of decrease in an output of said sensor due to a reaction of NOx and ammonia in said sensor, based on the NOx concentration estimated by said NOx concentration estimation unit and the ammonia concentration estimated by said ammonia concentration estimation unit; a diagnostic unit configured to carry out a diagnosis of said sensor based on an output value of said sensor; and a diagnostic control unit configured to control the diagnosis in said diagnostic unit based on the amount of decrease in the output of said sensor calculated by said calculation unit. an NOx concentration estimation unit configured to estimate a concentration of NOx in the exhaust gas flowing out of said NOx selective reduction catalyst; an ammonia concentration estimation unit configured to estimate a concentration of ammonia in the exhaust gas flowing out of said NOx selective reduction catalyst; a calculation unit configured to calculate an amount of decrease in an output of said sensor due to a reaction of NOx and ammonia in said sensor, based on the NOx concentration estimated by said NOx concentration estimation unit and the ammonia concentration estimated by said ammonia concentration estimation unit; a diagnostic unit configured to carry out a diagnosis of said sensor based on an output value of said sensor; and a diagnostic control unit configured to control the diagnosis in said diagnostic unit based on the amount of decrease in the output of said sensor calculated by said calculation unit.