PURPOSE: To enhance the accuracy of detection for an exhaust gas recirculation rate to enhance the accuracy of control and to eliminate variations in the output signal of an oxygen sensor, by provding such an arrangement that the stream of intake-air is divided, there are formed first and second intake-air passages, exhaust gas is recirculated into the second passage and an oxygen sensor is provided to control the exhaust gas recirculation rate.

CONSTITUTION: Intake-air for an engine 1 flows into first and second intake-air passages 3, 6. Intake-air flowing through the passage 3 is mixed with fuel in a fuel supply device 5 and enters in the engine 1. Intake-air flowing through the passage 6, flows through a throttle valve 7, and flows into an intake-air pipe 6 after being mixed with recirculated exhaust gas at an exhaust gas introduction port 8. An oxygen sensor 9 detects the mixed rate of intake-air and exhaust gas in the passage 6. Variations in output voltage with respect to the exhaust gas recirculation rate becomes lager in accordance with the area ratio of the first and second passages, and therefore detection may be made with a high degree of sensitivity. Further, no fuel is made into direct contact with the oxygen sensor which comes to be a high temperature so that there is no risk of firing of fuel, and variations in the output signal due to fuel vapor may be eliminated.