An improved rocker arm which is held in positive engagement with a head thrust surface of a cylinder head to avoid the occurrence of pitching of a valve stem end and vibration or noise from a valve-driving system. The rocker arm enables a rocker shaft spring to be set at a reduced load, whereby low friction is realized. An arm thrust surface of the rocker arm is provided with an oil groove having a predetermined depth. The arm thrust surface is positioned against a head thrust surface of the cylinder head. The oil groove communicates with the rocker shaft hole so as to introduce oil outside.
