PURPOSE: To improve the reliability of decision result of an exhaust gas temperature abnormality detecting device for detecting the abnormality from the exhaust gas temperature of an internal combustion engine.

CONSTITUTION: As the exhaust gas temperature of an internal combustion engine is largely influenced by the change of the output (load) of the internal combustion engine and the ambient temperature, three data, a load signal P from a load signal detector 2 for detecting the load, an ambient temperature signal TA from an ambient temperature detector 3 for detecting the ambient temperature of the internal combustion engine and an exhaust gas temperature signal TEX from an exhaust gas temperature detector 4 for detecting the exhaust gas temperature of an internal combustion engine are input to CPU 1. The CPU 1 calculates an estimated exhaust gas temperature from the above three data according to the data stored in RAM to be compared with the above exhaust gas temperature signal TEX and examined. Thus, normality /abnormality is decided, and the decision result is output to a decision result display device 7.