PURPOSE: To drastically facilitate the balance working by locally forming non- austempering treated parts on the balance weight part of a crankshaft to be subjected to an austempering treatment and rendering the material easy to work.

CONSTITUTION: A heat insulating jig 4, e.g., is fixed on the balance weight part 3 of the crankshaft M worked into the prescribed shape. The cooling ratio in the fixed part of the jig 4 is slowed at the time of subjecting said shaft M to the austempering treatment. High strength and high toughness are then provided on the surface of the shaft M except the fixed part of the jig 4. The non-austempering treated parts are locally formed on the part fixed with said jig 4 as well to facilitate the working.