A fuel injection system for an internal combustion engine has a lubricity improver supply for adding a lubricity improver to low-viscosity fuel supplied to a fuel injection apparatus of the engine. A controller controls the lubricity improver supply so that the amount of the lubricity improver added to fuel at the inlet to the fuel injection apparatus is increased as engine rotation speed, engine load, or injection pressure is increased. With the system, wear or sticking of the plunger can be prevented even when low-viscosity fuel is used by improving lubrication conditions to secure necessary lubrication condition for the sliding part in accordance with engine operation conditions, fuel temperature and viscosity.