A two-stroke engine having an intake valve and an exhaust valve which are arranged on the cylinder head. The peripheral portions of the inner wall of the cylinder head around the intake and the exhaust valves are cut away, and the peripheral portions of the top portion of the inner wall of the cylinder bore around the intake and the exhaust valves are cut away. A masking wall is formed on the inner wall of the cylinder head to mask the valve opening between the valve seat and the peripheral portion of the intake valve, which is located on the exhaust valve side, for the entire time for which the intake valve is open.