In an engine on a boss for fastening a cylinder cap cover integrally arranged on a cylinder cap with a bolt and screw thread inserted and penetrating through the cylinder cap cover for fastening the cylinder cap cover by the butt-joint of a gasket and a junction surface at the upper portion of the cylinder cap at the lower part of the cylinder cap cover, the bolt for fastening the cylinder cap cover can be shortened, the assembling performance can be improved, simultaneously the engine is light. According to the invention, the boss (96) arranged above the junction surface ( 61) for fastening the cylinder cap cover forms a space under the boss (96), and is integrally arranged at the upper part of a cam shaft holder (68) integrally mounted on the cylinder cap (39) in the way that one part ofcam shaft holder (63) for forming a valve transmission mechanism ( 57) can be freely rotatablely supported, in the way that the boss protrudes from cam shaft holder (68) to the side.