A multistage fuel injection system for an internal combustion engine has an injection nozzle capable of injecting main and auxiliary fuels into a cylinder of the engine at controlled intervals, thereby completely combusting an unburned fuel-and-air mixture in the cylinder. The injection nozzle includes a main injection passage for the main fuel, a needle valve for opening and closing the main injection passage, an auxiliary injection passage for the auxiliary fuel communicating with the main injection passage through the needle valve at a discharge end of the latter, and a check valve for preventing a reverse flow of the main fuel from the main injection passage to the auxiliary injection passage.