A fuel injection system for an internal combustion engine which is designed to determine an instantaneous engine speed in a cycle of, for example, 30° CA of the engine to learn an actual injection quantity that is the quantity of fuel actually sprayed from a fuel injector. The system filters the instantaneous engine speed using a band-pass filter to extract a cyclic component which varies in synchronism with an engine operating cycle to produce an engine speed change which has arisen from the spraying of fuel into the engine from which unwanted noise components are removed and uses it to determine the actual injection quantity.