A camshaft of a valve operating system for an internal combustion engine, is formed of iron-based alloy casting and has cam sections, each of which is in slidable contact with a heat resistant tip of a rocker arm. The cam section is formed at its surface portion with a surface hardened layer formed of an air-cooled chilled structure. The surface hardened layer is formed by remelting a part of the surface of the casting after preheating. An intermediate hardened layer of a heat affected zone is formed in contact with the surface hardened layer and has a thickness ranging from 0.5 to 2.0 mm. The intermediate hardened layer is formed of a mixed structure of bainite transformation phase and troostite and is formed by being thermally affected during the remelting.