In an engine having a camshaft mounted on a cylinder head via plural camshaft supporting portions; a camshaft sprocket coaxially coupled to the camshaft; an idler sprocket rotatably supported between the camshaft and the crankshaft on the cylinder head; a first chain reeved around the crankshaft and the idler sprocket; and a second chain reeved around the idler sprocket and the camshaft sprocket, an oil jet hole is formed in the frontmost camshaft supporting portion in communication with a main oil passage via an annular oil groove formed in the camshaft and an inclined oil groove formed in the cylinder head, so that lubricant can be sufficiently jetted from the oil jet hole to the second chain located near the frontmost camshaft supporting member, without use of an additional oil jet pipe member.