PURPOSE: To reduce annealing cost by performing mechanical machining without applying annealing if the difference of margin between respective cams is smaller than a predetermined value.

CONSTITUTION: Cam sections 2a, 2bW5a, 5b of a cast iron cam shaft 1 are chill hardened. Margins required for respective cams under cast state of the iron cast camshaft 1 are obtained, and if the difference of margin between cams having phases shifted by 180° is larger than a predetermined value, annealing is performed followed by mechanical machining. If the difference is smaller than the predetermined value, mechanical machining is performed directly thus reducing manufacturing cost.