PURPOSE: To make the width and depth of hardening uniform when the surfaces of the cams of a cast iron camshaft are hardened, by carrying out successive hardening so that adjacent cams are not continuously treated.

CONSTITUTION: The surfaces of cams of a camshaft having many cams are hardened by remelting with melting energy. Untreated cams which are not adjacent to the treated cams and have not been affected by heat during the hardening are then selected and the surfaces of the selected cams are hardened. Thus, successive hardening is carried out.