PURPOSE: To obtain a small and light internal combustion engine, by disposing pistons at the opposite end portions of a semi-circular cylinder in a freely slidable manner, connecting the two pistons together by a piston rod, and transmitting motion of te two pistons to a crankshaft.

CONSTITUTION: A cylinder 1 is shaped in a semi-circular, cylindrical form and pistons 2 are disposed at the opposite end portions of the cylinder 1. The two pistons 2 are connected together by an arcuate piston rod 3, the intermediate portion of which is connected to a crankshaft 5 by a connecting rod 4. On the other hand, ignition plugs 7 and intake and exhaust valves 8 are provided in a cylinder head, and the valves 8 are opened and closed by respective rocker arms 10 through rotation of a cam shaft 9. With such an arrangement, the crankshaft 5 is turned by the piston rod 3 and the connecting rod 4 through different-phase motions of the two pistons 2.