Abstract: The present invention is an internal combustion engine having a rotary sleeve valve mechanism with a sleeve valve which has an opening formed in the outer peripheral surface for suction of fuel and discharge of exhaust gas. The present invention achieves an improvement in the admission and exhaust efficiency and simplification of the valve mechanism by employing a rotating sleeve valve. The rotary sleeve-valve internal combustion engine has a cylindrical rotary cylinder valve (3) which is rotatably supported in an engine block (1). An opening (5) which is provided in the outer peripheral wall surface of the rotary cylinder valve (3) provides communication with an inlet port (10) during admission and with an exhaust port (15) during exhaust. A seal ring (40) is provided around the periphery of the opening (5) to effect gas seal for the area between the engine block (1) and the inner peripheral wall surface (7).