To miniaturize an assist gas fuel injection mechanism by reducing a clearance between a valve element of a rotary valve and a housing as much as possible and reducing the leakage of the injection assist gas in the assist gas fuel injection mechanism for a 2-cycle internal combustion engine providing a chamber communicated with the combustion chamber via a communication passage, the rotary valve controlling its opening/closing in the communication passage, a fuel injector between the rotary valve of the communication passage and the chamber, mixing high compressed gas from the chamber with the fuel from the fuel injector, and feeding thick fuel-air mixture to the fuel chamber.

This assist gas fuel injection mechanism for the 2-cycle internal combustion engine is characterized that a protective layer is formed on the outer surface of the valve element of the rotary valve using a material softer, and easier of plastic deformation than those of the valve element of the rotary valve and the housing of the rotary valve.