A method of producing a cam lobe piece of an assembled camshaft in a valve operating system for an internal combustion engine. The method comprises (a) forming a profile of the cam lobe piece by upsetting a material under forging to obtain an intermediately formed body; (b) piercing a central portion of the intermediately formed body to form a shaft bore; and (c) ironing an inner peripheral surface of the pierced intermediately formed body to form unevenness at the inner peripheral surface, all accomplished by cold working. The material at the forming the profile of the cam lobe piece has a first section located on a side of a cam nose of the cam lobe piece, and a second section located longitudinally opposite to the first section. The material has a thickness which gradually increases in a direction from the second section to the first section.