A method of producing a cast iron camshaft including the steps of applying a high-density energy source, such as a TIG arc, a laser beam, and an electron beam over the entire sliding surface of a cam to melt the surface portion and cooling the cam by dissipating the localized heat through the camshaft itself so as to form a double structure hardened layer consisting of a chilled layer having a large amount of cementite and a hardened layer of martensite. This method omits the preheating of the camshaft prior to the high-density energy irradiation.