A simple structure is adopted to reduce the volume in a crank case and facilitate a scavenging efficiency. When a piston 11 is located on the side of a bottom dead center, at a part corresponding to a notch part in the piston 11, an inner surface of the crank case 20 protrudes toward an interior side to form a crank case first protruding part 20A penetrating through the notch part toward the interior side. Meanwhile, the crank case 20 at a part corresponding to a part without the notch part in the piston 11 is not in a shape protruding toward the interior side. By assembling the piston 11 and the crank case 20 in such shapes, the piston 11 is able to reciprocally move up and down without interfering with the crank case 20. In this case, by disposing the crank case first protruding part 20A, the volume in the crank case 20 is able to be reduced.
