Abstract: A fuel injection system of an internal combustion engine including air introducing passage device extending in one direction for introducing auxiliary air under pressure, annular fuel passage of a disc-like space extending radially outwardly from the air introducing means and substantially perpendicularly to the air introducing passage for introducing fuel for combustion, annular opening provided at inner end of the annular fuel passage for injecting the fuel into the auxiliary air flowing in the air introducing passage from position surrounding the auxiliary air flow at a right angle to the flowing direction of the auxiliary air so that the fuel is atomized to be mixed with the auxiliary air. The fuel from the fuel injecting means is substantially perpendicularly injected into the auxiliary air flow in the form of a thin membrane so that the atomization of the fuel can be effectively facilitated resulting in an improved combustion performance of the liquid fuel.