An assist gas fuel injection mechanism for a 2-cycle internal combustion engine includes a chamber communicated to a combustion chamber through a communicating passage. A rotary valve is provided in the communicating passage for controlling the opening and closing of the communicating passage. A fuel injection device is provided in the communicating passage between the rotary valve and the chamber. With this configuration, a highly compressed gas from the chamber, and fuel from the fuel injection device, are mixed with each other, and a concentrated mixture gas is supplied to the combustion chamber through the communicating passage. A protective layer is provided on the outside surface of the valve body of the rotary valve by using a material which is softer than the valve body of the rotary valve and the housing of the rotary valve and which is susceptible to plastic deformation.