A fuel injection system for diesel engines, capable of carrying out a control operation for varying a mixing ratio of a fuel to water (inert material) arbitrarily in accordance with the magnitude of an engine load in a short period of time, and holding down the formation of nitrogen oxides to a level equal to that in a related art fuel injection system using an aqueous emulsion fuel, by uniformly mixing the fuel in the same manner as an aqueous emulsion fuel used in a related art fuel injection system, with water (inert material) in the combustion chamber. A fuel is injected from a fuel injection nozzle toward the water (inert material) held in water holding portions, or a fuel is injected from a fuel injection nozzle into the water (inert material) supplied from water supply passages.