PURPOSE: To constitute a hollowed recess extremely easily by setting a die corresponding to the shape of the hollowed recess into a sand mold after applying antiburn-on treatment on the casting surface thereof in forming the hollowed recess to a crank arm surface close to the revolving center of a shaft.

CONSTITUTION: In constituting an engine crank shaft, two pieces a pair of crank arms 3 are provided successively in continuation to a journal 1 concentrical to the axial center of the crank, and these are connected by means of a crank pin 2 to provide arm corner parts (a) and balance weights 4, thence the crankshaft is cast. At this time, a hollowed recess 5 which relieves the concentration of tensile stress is formed to the arm 3 surface in proximity to the pin of the part near the rotating center of the crank shaft at the connected part of the crank pin 2 and the crank arm 3. In forming said part 5, a die 6 bulged and formed with a raised part 6a of a partially cylidrical shape corresponding to said recess is made and in order to prevent melt-sticking to molten metal, a mold coating material is coated thereon, thence it is set in the prepared position of the sand mold and casted.