An abnormality determination device for an internal combustion engine, the internal combustion engine including a NOx purification catalyst and a reducing agent supply mechanism, the NOx purification catalyst being disposed in an exhaust passage of the internal combustion engine, the NOx purification catalyst being configured to purify the NOx in the exhaust gas, the reducing agent supply mechanism being disposed on an upstream side of the NOx purification catalyst in the exhaust passage, and the reducing agent supply mechanism adding urea water to the exhaust gas flowing through the exhaust passage such that ammonia originating from the urea water is supplied to the NOx purification catalyst as a reducing agent, the abnormality determination device comprising: a NOx sensor configured to detect an amount of the NOx contained in the exhaust gas flowing through a downstream side of the NOx purification catalyst in the exhaust passage; and an electronic control unit configured to determine, as an abnormality determination processing, that a concentration of the urea water is an abnormal concentration in a case where a difference between a first NOx amount and a second NOx amount detected by the NOx sensor is equal to or lower than a predetermined value, the first NOx amount being the amount of the NOx detected by the NOx sensor during non-execution of an addition of the urea water by the reducing agent supply mechanism, the second NOx amount being the amount of the NOx detected by the NOx sensor during execution of the addition of the urea water by the reducing agent supply mechanism, characterized in that, in addition, the electronic control unit is configured to determine, as an abnormality determination processing, that a concentration of the urea water is an abnormal concentration in a case where a difference between a first exhaust gas temperature and a second exhaust gas temperature exceeds a predetermined temperature, the first exhaust gas temperature being a temperature of the exhaust gas during the non-execution of the addition of the urea water by the reducing agent supply mechanism, and the second exhaust gas temperature being the temperature of the exhaust gas during the execution of the addition of the urea water by the reducing agent supply mechanism, the first and second exhaust gas temperatures being measured by an exhaust gas temperature sensor disposed downstream to the NOx purification catalyst. a NOx sensor configured to detect an amount of the NOx contained in the exhaust gas flowing through a downstream side of the NOx purification catalyst in the exhaust passage; and an electronic control unit configured to determine, as an abnormality determination processing, that a concentration of the urea water is an abnormal concentration in a case where a difference between a first NOx amount and a second NOx amount detected by the NOx sensor is equal to or lower than a predetermined value, the first NOx amount being the amount of the NOx detected by the NOx sensor during non-execution of an addition of the urea water by the reducing agent supply mechanism, the second NOx amount being the amount of the NOx detected by the NOx sensor during execution of the addition of the urea water by the reducing agent supply mechanism,