PURPOSE: To perform air-fuel ratio control accurately in spite of deterioration in an oxygen sensor, by varying a control constant when each peak difference between both rich and lean sides of output of the oxygen sensor in judged as there is the specified reduction as compared with a value of the last difference.

CONSTITUTION: At the time of engine running, conditionality of whether an operational judgment of a difference between both peak output values Vr and Vl at both rich and lean times is performed at an operational judging circuit 7 from a fundamental injection quantity (d) operated by an operational circuit 4 from each output, a and b, of an engine speed sensor 1 and an air flow meter 2 and an engine speed (a) so not is carried out. When a running condition for performing this judgment is given, operation of (Vr-Vl) is performed by an operational circuit 11, and the result (k) is compared with the value Vo stored in a memory 12 with a comparator 13. And, corresponding to the compared signal l, a PI constant of a feedback PI signal is altered by a constant altering circuit 14, and this is stored in a register 15, while the altered PI constant is utilized for an injection quantity calculation.