In a process of mounting a camshaft 20 to a supporting member 10, a shaft body 21 is firstly inserted into a first bearing hole 13F in the front side of the supporting member 10, before the shaft body 21 is axially moved toward the second bearing hole 13R. In order to get a cam lobe 22 through a concave bearing portion 13M in a semicircular arc shape, rotation of the camshaft 20 enables a cam nose 22b to be directed downward, which is opposite from the concave bearing portion 13M. This enables avoiding the interference between the cam nose 22b and the concave bearing portion 13M, even when the curvature radius of the concave bearing portion 13M is reduced. Consequently, reduction of the curvature radius of the concave bearing portion 13M achieves the downsizing of the supporting member 10.