A fuel injection amount control apparatus of an embodiment according to the present invention comprises an air-fuel ratio sensor 56 which is disposed between an exhaust gas aggregated portion HK and a three-way catalyst 43, and which outputs an output value corresponding to an amount of oxygen and an amount of unburnt substances contained in an exhaust gas that has reached an exhaust-gas-side electrode layer via a porous layer (diffusion resistance layer). This control apparatus obtains an actual detected air-fuel ratio by correcting the output value of the air-fuel ratio sensor 56 based on an alcohol concentration of a fuel detected by the alcohol concentration sensor 59 and an air-fuel ratio imbalance indicating value representing a degree of a non-uniformity among cylinder-by-cylinder air-fuel ratios. The control apparatus feedback controls on a fuel amount in such a manner that the actual detected air-fuel ratio coincides with a target value.