To provide a fuel injection system for an internal combustion engine capable of stable fuel injection control by preventing fuel mixing when switching-over a fuel, in an internal combustion engine using two types of fuel.

The fuel injection system for an internal combustion engine provides a fuel injection by supplying two types of fuel or liquid fuel and gaseous fuel selectively to a set of fuel injection valves 4. The fuel injection valves 4 have a liquid fuel supply port 43 for supplying liquid fuel and a gaseous fuel supply port 44 for supplying gaseous fuel. A shutoff valve 8 is disposed in a supply line connecting to the gaseous fuel supply port 44. A check valve 25 is disposed in a supply path of liquid fuel in communication with the liquid fuel supply port 43 in the fuel injection valve 4. When a pressure in a valve chamber of the fuel injection valve 4 drops below a supply pressure of liquid fuel, the liquid fuel opens the check valve 28, which is turn introduces it into the valve chamber 42 from the liquid fuel supply port 43.