NOX SENSOR DIAGNOSING DEVICE AND SCR SYSTEM

Provided are a NOx sensor diagnosis apparatus capable of diagnosing the appropriateness of a NOx sensor by considering environmental conditions and preventing misdiagnosis, and an SCR system. A NOx sensor diagnose apparatus, which diagnoses a NOx sensor 110 provided in an exhaust pipe 102 of an engine E and configured to detect the concentration of NOx in exhaust gas, includes a calculation value operating unit 2 configured to acquire the calculation value of the concentration of NOx in the exhaust gas based on the actuation state of the engine E, an error determining unit 3 configured to determine that the NOx sensor 110 is erroneous when a difference between the calculation value of the concentration of NOx acquired by the calculation value operating unit 2 and the detection value of the concentration of NOx detected by the NOx sensor 110 is larger than a determination threshold value, and a determination threshold value setting unit 4 configured to set the determination threshold value based on the temperature of the cooling water for cooling the engine E, an atmospheric pressure, and an outdoor temperature.