To provide a two-cycle internal combustion engine wherein the fuel passage of a carburetor is constituted simply and an air fuel ratio is not switched in a middle speed range of the engine.

An intake port 25b and an air entrance 24 opening to a crank room 13a and an injection port 22 opening to a combustion chamber 4 are provided in the cylinder wall 3 of the engine. A rotary throttle valve 39 having throttle holes 36 and 40 is inserted into a cylindrical valve chamber 35, which intersects the fuel passage 21 and the intake passage 25 provided in parallel with a carburetor body 34, so as to be turnable and movable vertically. A needle 37 extending downward from the throttle valve 39 is inserted into a fuel supply pipe 20 fittedly, which extends from the combustion chamber 47 of the body 34 to the throttle hole 36 through the shaft center of the throttle valve 39, to adjust the opening of a fuel injection hole 38. A first passage 21b connecting the fuel passage 21 and an injection port 33 and a second passage 25a connecting an intake passage 25 and the intake port 25b are provided in the connection member 18 interposed between the body 34 and the cylinder wall 3, and the first passage 21b and the second passage 25a are connected to each other through an injection pipe 28.