An abnormality detecting device for an EGR system comprises an EGR valve disposed in an EGR passage to control a flow rate of exhaust gas recirculated therethrough, a first temperature detecting device disposed in the EGR passage to detect the temperature of the same, a second temperature detecting device disposed in the intake air passage of an internal combustion engine, an operational condition detecting device to output parameters of the engine or the EGR valve as signals indicating operational conditions of the engine, an EGR abnormality determining zone discriminating device adapted to discriminate that the operational conditions are in a predetermined EGR abnormality determining zone in the operable area of the engine or the EGR valve in which recirculation of the exhaust gas is controlled by the EGR valve, and an abnormality determining device to determine abnormality in the EGR system on the basis of a quantitative relation between a value of the EGR abnormality determining temperature which is obtained by the calculation of the operational conditions and an output signal of the second temperature detecting device, and a value of the temperature of the EGR passage detected by the first temperature detecting device.