To improve quality of a recirculating exhaust gas temperature signal diagnosis device for an exhaust gas recirculation system by taking consideration to an exhaust gas recirculation condition so as to reduce wrong diagnosis of the temperature signal.

It is judged whether sensors for detecting various conditions for a diagnosis operate normally (S1), an engine is not in starting period (S2), the engine is in an idle state (S3), and the cooling water temperature is in a prescribed temperature range (S4). After that, in case where exhaust gas recirculation is executed (S5), when the intake air temperature of an intake air is in a prescribed range (S6) and such a condition that the detected recirculating exhaust gas temperature is the prescribed value α or less continues for a prescribed time, the temperature signal is judged to be abnormal (S7-9). On the other hand, in case where exhaust recirculation is not executed (S5), and when such a condition that the detected recirculating exhaust gas temperature is higher than the prescribed value continues for the prescribed time, the temperature signal is judged to be normal (S11-13).