PURPOSE: To perform the calculation of a fundamental fuel injection quantity from a map at time of EGR system operation irrespective of the presence of exhaust gas recirculation by setting a fuel supply according to oxygen partial pressure in suction air at the downstream of a throttle valve so as to become the optimum air-fuel ratio at the time of EGR system operation.

CONSTITUTION: Each detection value of crank angle sensors 56 and 58, an exhaust side oxygen sensor 68, a suction side oxygen sensor 70 being installed at the downstream of a throttle valve 34 and detecting oxygen partial pressure of suction air, a water temperature sensor 64, a suction air temperature sensor 66, etc., is inputted into a control circuit 55. The control circuit 55 sets a fundamental fuel injection quantity so as to become a theoretical air-fuel ratio to fresh air volume to be led into an internal combustion engine in response to the detection value of the suction side oxygen sensor 70 irrespective of operation or non operation of an EGR valve 46.