To prevent the concentration of stresses by joining metallic plates, manufacturing a hollow body having similarily smaller than a casting shape, using a hollow metallic core communicating gas venting small diameter pipes, casting, cutting off and removing the small diameter pipes and closing with a plug.

The metallic plates 1, 2 are joined with a press or a welding and the hollow metallic core 6 having almost similarily smaller outer shape than the shape of a metallic casting frame W and the hollow part 3 is manufactured. The metallic plate 2 is formed as rugged surface so as to absorb the thermal expansion at the time of casting and the shrinkage at the time of solidifying. The small diameter pipes 4 for gas venting are communicated and fixed to the outside of the hollow metallic core 6. The hollow metallic core 6 is set in a sand mold 7 and the casting executed by pouring molten metal. After completing the casting, the small diameter pipes 4 for gas venting are cut off along the end surfaces of the whole casting frame W and screw holes are worked to the remained small diameter pipe 4 parts and bolts are buried to close the holes by plug-welding. By this method, the communicating parts become the small diameters and the concentration of stresses can be prevented.