A fuel injection system is proposed for Diesel type internal combustion engines, by means of which the ignition conditions of a Diesel engine are kept optimal even during operation at high altitudes despite a reduced air charge. The system includes a fuel injection pump equipped with an injection timing adjustment apparatus which functions in accordance with rpm and a control apparatus functions in accordance with the air quantity supplied to the engine. By means of the control apparatus, a correction controlled in accordance with air pressure and therefore dependent on air quantity is superimposed on the change in the onset of the fuel supply controlled by the injection adjuster at least in accordance with rpm. When a hydraulic injection adjuster is used, the supply pressure of a control pump is increased by a correcting final control element of the control apparatus when the air quantity is decreasing in the direction toward an early adjustment of the onset of supply.