PURPOSE: To diagnose an EGR device with high precision by forcing an EGR valve to open or close so that a change in combustion pressure (cylinder internal pressure) is forcibly brought about, and making a diagnosis on whether or not there is failure according to whether or not combustion pressure change matching the control of opening or closing the valve has actually occurred.

CONSTITUTION: An EGR valve 5 inserted in an EGR passage 4 communicating the exhaust manifold 2 with the intake manifold 3 of an engine 1 is opened by introducing the negative intake pressure of the engine 1 into a pressure chamber via a negative pressure inlet passage 7. An EGR control solenoid 9 is inserted in the negative pressure inlet passage 7 and is controlled by a control unit 8 in accordance with outputs of a cylinder internal pressure sensor 11, etc. For the diagnosis of such an EGR device, the deviation of integrated values of cylinder internal pressure when the EGR valve 5 is forced to open or close is calculated. On the basis of this deviation and a deviation preset to suit the operating conditions, it is determined whether or not the EGR device has failed.