An internal combustion engine includes a cylinder block having a cylinder bore, a cylinder head having a combustion chamber communicating with the cylinder bore, a spark plug supported in the cylinder head and having electrodes disposed in the combustion chamber, two first valves supported on the cylinder head and having valve heads disposed in one side of the combustion chamber, and two second valves supported on the cylinder head and having valve heads disposed in an opposite side of the combustion chamber. A valve operating mechanism associated with the internal combustion engine comprises a camshaft rotatably mounted on the cylinder head and engaging the first valves for operating the first valves, a rocker arm holder mounted on the cylinder head, a rocker arm shaft or shafts disposed between the camshaft and the second valves and rotatably supported by the rocker arm holder, and two rocker arms swingably mounted on the rocker arm shaft or shafts and held in engagement with the camshaft and the second valves for operating the second valves. The rocker arm holder is disposed above the cylinder bore in substantial central alignment therewith, the rocker arm holder having a plug insertion hole for insertion of the spark plug therethrough.