A fuel injection system of the type in which the opening time of a fuel injection valve is controlled on the basis of a predetermined program and in accordance with various operation parameters such as the intake air flow rate, engine speed and engine temperature. The system has a means for detecting at least one of the offset of the engine speed from a command speed and the engine speed variation per unit time, the data being used together with the above-mentioned operation parameters in controlling the opening time of the fuel injection valve.