1. A system for controlling air/fuel ratios of individual cylinders of a multicylinder internal combustion engine connected to an exhaust pipe via an exhaust manifold that constitutes an exhaust system of the engine, said system having: an air/fuel ratio sensor installed at the exhaust system of the engine; detection circuit for receiving an output of said air/fuel ratio sensor to determine an air/fuel ratio at a confluence point of the exhaust system; modeling means for modeling a behavior of the exhaust system by an equation established based on an assumption that the air/fuel ratio at the confluence point is expressed as a sum of products of past firing histories of the individual cylinders and a weighting coefficient C each indicative of contribution thereto of the individual cylinders; estimating means for estimating the air/fuel ratios in the individual cylinders from the equation; fuel injection quantity determining means for determining a fuel injection quantity to be supplied to the engine at least based on a base value and the estimated air/fuel ratios of the individual cylinders; and a fuel injector for injecting fuel based on the determined fuel injection quantity; wherein the improvement comprises: zero setting means for setting the weighting coefficient C to zero for a cylinder other than most recent m (2.ltoreq.m&lt;n) cylinders when a number of cylinders to be estimated their air/fuel ratios is 3.ltoreq.n.