To eliminate overhung structure of a camshaft and maintain clearance between the camshaft and a tappet to be constant by forming the one end of the camshaft into a projecting part extending outside, and rotatably supporting the projecting part with a cam bearing part provided on a chain cover and a cam housing.

The intermediate part positions of a camshaft 12 which extends in the longitudinal direction of the cylinder head are rotatably supported with a plurality of bearings formed on the cylinder head of an engine, and through the rotation of the camshaft 12, intake and exhaust valves are opened/closed via a plurality of cams 16. In this case, the one end of the camshaft 12 is formed into a projecting part extending outside the outside of a sprocket 18 and an extended bearing part 24 is provided. Meanwhile, a chain cover 22 is provided with a chain cover part cam bearing 26, and the extended bearing part 24 is rotatably supported with the cam bearing 26 and a cam housing 10. Hereby the sprocket 18 mounted on the one end of the camshaft 12 is twin-supported, and the clearance between the camshaft 12 and the tappet 32 is maintained constant.