A lower cam carrier is disposed over a cylinder head and a head cover is disposed thereover. The lower cam carrier has, formed as one therewith, an outer frame superposed with the edge peripheral edge of the cylinder head, and a bridging part provided between opposing sides of the outer frame part. The bridging part has a lower bearing for supporting an intake camshaft and an exhaust camshaft. The head cover has, formed as one therewith, a flange part superposed with the outer frame part and a bearing part opposing the bridging part inside the flange part 22. The bearing part has an upper bearing that, together with the lower bearing, supports the intake camshaft and the exhaust camshaft.