PURPOSE: To obviate superfluous exhaust gas recirculations even if exhaust pressure vaires as well as even if an excess air ratio in a Diesel engine varies with a change of atmospheric pressure, by controlling an exhaust gas recirculating flow rate in proportion to the oxygen content of exhaust gases.

CONSTITUTION: According to comparative results between the oxygen content of exhaust gases measured by an oxygen content sensor 38 and the duty ratio control data corresponding to engine load and engine speed at that time or the desired oxygen content data, the duty ratio of a pulse signal outputted to a suction control valve 32 is controlled so as to cuase the measured oxygen content data to be equaled to the desired oxygen content data. Based on that controlled duty ratio, the suction pressure imparted to a pipe 33 is adjusted whereby the valve opening amount of an exhaust gas recirculation control valve 14 is controlled so that the amount of exhaust gas recirculating flow is controlled as desired.