An engine includes: a crankshaft 12; a camshaft 36; and a timing transmission system 37 which has a drive rotation member 45, a driven rotation member 46, and an endless power transmission member 47, and which provides connection between the crankshaft 12 and the camshaft 36. An access window 55 through which the driven rotation member 46 is attached to and detached from the camshaft 36 is opened in an outer end surface 5c of the cylinder head 5. A lid body 57 for closing the access window 55 is jointed to the outer end surface 5c. The outer end surface 5c of the cylinder head 5 comprises a slanted surface 5c which is inclined so that at least a part of an outer periphery of the driven rotation member 46 on a side opposite from the drive rotation member 45 is exposed from the access window 55. Thus, it is possible to facilitate the operation of attaching the endless power transmission member to the driven rotation member and mounting the driven rotation member to the camshaft, and also contributing to downsizing of the engine.