A deterioration determination system for an exhaust emission control device including: a selective reduction type NOx catalyst provided in an exhaust path of an internal combustion engine and using ammonia as a reducing agent; a reducing agent supply unit that supplies ammonia or a precursor of the ammonia into an exhaust gas flowing into the selective reduction type NOx catalyst upstream of the selective reduction type NOx catalyst; a catalyst provided downstream of the selective reduction type NOx catalyst and having an oxidative capacity; an NOx sensor provided downstream of the catalyst having the oxidative capacity to detect NOx in the exhaust gas flowing out from the catalyst having the oxidative capacity and also to detect the ammonia in the exhaust gas as NOx; and a deterioration determination unit that makes a deterioration determination of the selective reduction type NOx catalyst based on a detection value of the NOx sensor, characterised in that the deterioration determination system comprises and is configured such that: an outflow amount calculation unit that calculates, when an amount of the ammonia adsorbed by the selective reduction type NOx catalyst is larger than an equilibrium adsorption amount which is an adsorption amount of the ammonia when adsorption and release of the ammonia by and from the selective reduction type NOx catalyst are in an equilibrium state, out of the amount of the ammonia flowing into the catalyst having the oxidative capacity, at least one of the amount of the ammonia flowing out from the catalyst having the oxidative capacity and the amount of the ammonia converted to NOx in the catalyst having the oxidative capacity or a total value of both; and a deterioration determination limiting unit that, when the value calculated by the outflow amount calculation unit exceeds a threshold, limits use of the detection value of the NOx sensor in the deterioration determination by the deterioration determination unit, or prohibits the deterioration determination itself by the deterioration determination unit. a selective reduction type NOx catalyst provided in an exhaust path of an internal combustion engine and using ammonia as a reducing agent; a reducing agent supply unit that supplies ammonia or a precursor of the ammonia into an exhaust gas flowing into the selective reduction type NOx catalyst upstream of the selective reduction type NOx catalyst; a catalyst provided downstream of the selective reduction type NOx catalyst and having an oxidative capacity; an NOx sensor provided downstream of the catalyst having the oxidative capacity to detect NOx in the exhaust gas flowing out from the catalyst having the oxidative capacity and also to detect the ammonia in the exhaust gas as NOx; and a deterioration determination unit that makes a deterioration determination of the selective reduction type NOx catalyst based on a detection value of the NOx sensor, the deterioration determination system comprises and is configured such that: an outflow amount calculation unit that calculates, when an amount of the ammonia adsorbed by the selective reduction type NOx catalyst is larger than an equilibrium adsorption amount which is an adsorption amount of the ammonia when adsorption and release of the ammonia by and from the selective reduction type NOx catalyst are in an equilibrium state, out of the amount of the ammonia flowing into the catalyst having the oxidative capacity, at least one of the amount of the ammonia flowing out from the catalyst having the oxidative capacity and the amount of the ammonia converted to NOx in the catalyst having the oxidative capacity or a total value of both; and a deterioration determination limiting unit that, when the value calculated by the outflow amount calculation unit exceeds a threshold, limits use of the detection value of the NOx sensor in the deterioration determination by the deterioration determination unit, or prohibits the deterioration determination itself by the deterioration determination unit. an outflow amount calculation unit that calculates, when an amount of the ammonia adsorbed by the selective reduction type NOx catalyst is larger than an equilibrium adsorption amount which is an adsorption amount of the ammonia when adsorption and release of the ammonia by and from the selective reduction type NOx catalyst are in an equilibrium state, out of the amount of the ammonia flowing into the catalyst having the oxidative capacity, at least one of the amount of the ammonia flowing out from the catalyst having the oxidative capacity and the amount of the ammonia converted to NOx in the catalyst having the oxidative capacity or a total value of both; and a deterioration determination limiting unit that, when the value calculated by the outflow amount calculation unit exceeds a threshold, limits use of the detection value of the NOx sensor in the deterioration determination by the deterioration determination unit, or prohibits the deterioration determination itself by the deterioration determination unit.