An engine air-fuel ratio control system uses a rich air-fuel ratio immediately after starting an engine to converge rapidly the air-fuel ratio toward a stoichiometric value. Upon determining an air-fuel ratio sensor is active, a stabilization fuel quantity increasing factor of a target air-fuel ratio revising coefficient decreases at a higher rate than before the air-fuel ratio sensor was active. Air-fuel ratio feedback control starts when the air-fuel ratio corresponds to a stoichiometric air-fuel ratio. Afterwards, when either air-fuel ratio feedback control starts or the engine enters a high rotational speed/high load region that operates using a rich air-fuel ratio, whichever occurs first, an unburned fuel quantity compensating value is set based on the stabilization fuel quantity increasing factor in effect at that point in time and added to the target air-fuel ratio revising coefficient while, simultaneously, the stabilization fuel quantity increasing factor is set to zero.