The present invention relates to an exhaust gas recirculation system which prevents an EGR sensor installed in an engine's EGR path (9) from being soiled by soiling substances in exhaust. A catalyst (23, 24, 7) is positioned upstream of the EGR sensor (12) to purify the soiling substances in EGR gas. The status of the EGR sensor (12) is properly controlled in accordance with the conditions of the catalyst (23, 24, 7). This ensures the EGR sensor (12) is less likely to be affected by the soiling substances in the exhaust under any circumstances of the EGR path (9) and allows the EGR sensor (12) to avoid a decrease in the accuracy of EGR gas detection. An alternative configuration may be employed such that the EGR gas recirculates from the downstream side of a catalyst (7) installed in a main exhaust path.