A fuel supply control apparatus for an internal combustion engine, which, when an air quantity detected at a predetermined crank angle of the internal combustion engine is represented by Qa and the (n-1)th air intake quantity and the next (n)th air intake quantity of the internal combustion engine at the predetermined crank angles thereof by Qe(n-1) and Qe(n) respectively, judges the optimum fuel supply quantity of the internal combustion engine by an equation: Qe(n)=K.multidot.Qe(n-1)+(1-K).multidot.Qa with filtering using the filter constant K, and which varies the filter constant K corresponding to an operating condition of the internal combustion engine, for example reduces K during idling, so that the fuel supply quantity can be controlled most suitably during idling and loaded operation.