PURPOSE: To easily discriminate a trouble on a sensor under an operating condition, by comparing temperature of EGR gas with an allowable value determined corresponding to an operating condition in case a feedback control is conducted on an EGR factor computed from a suction temperature, an EGR gas temperature, and temperature of mixture gas of the two gases so that the EGR factor attains a required value.

CONSTITUTION: A controller 6 computes an EGR factor based on values detected by an intake air temperature sensor 12, an EGR gas temperature sensor 13, and a sensor 14 for temperature of mixture gas of intake air and EGR gas, and a feedback control is made on the EGR factor through electromagnetic valves 15 and 17 so that the EGR factor attains a required value decided based on values detected by a sensor 8 for the number of revolutions and a load sensor 9. An allowable value deciding device 20 computes a lower limit value S3 and an upper limit value S4, being an allowable value of an EGR gas temperature, from the values detected by the sensor 8 for the number of revolutions and the load sensor 9. When a detected value S1 of the sensor 13 exceeds said allowable value, compractors 23 and 24 decide that either of the sensor 8 for the number of revolutions and the load sensor 9 is brought into a trouble, and light is generated by LED 31 and 32 to perform an alarm.