PURPOSE: To enable accurate fault diagnosis by setting a specified delay time after the operational condition for exhaust gas recirculation has been set, and memorying the maximum value detected by a temperature sensor during the period of said delay time so as to compare said maximum value with the set value for making fault dignosis.

CONSTITUTION: A temperature sensor 10 is installed on the midway of an exhaust circulation passage 6 for an EGR device communicating the way between an intake and an exhaust manifold 3 and 4, and the detected signal thereof is inputted into a control unit 15 together with detected singals from a throttle opening sensor 11, an engine revolving speed sensor 12, a water temperature sensor 13 and a solenoid valve operating sensor 14. And when it is judged at an EGR condition establishment judging section 16 that the EGR condition has been established, the detected signal from the temperature sensor 10 is inputted into a temperature detecting section 17 so as to memorize the maximum value during the period of the time set by a delay time setting section 18. Then, the aforesaid maximum temperature is inputted into a fault judging section 19, the judgement is formed that failures occur when the maxi mum temperature is less than the set value, thereby allowing a check lamp 20 to be lighted.