PURPOSE: To form a water-jacket part with using any cores and to improve the productivity by casting in the top part of water-jacket by a top-deck after casting an open-deck type cylinder block.

CONSTITUTION: The open-deck type cylinder block SO0 is molded. Next, the molten metal 14 is poured in the mold TD at the prescribed quantity, and promptly the open-deck type cylinder block S0 is made to over-turned condition and the top part thereof is set at the prescribed position in the mold TD. Then, gas generated in the closed water jacket part 6 is exhausted through a gas exhaust hole 13. After that, the molten metal 14 is cooled and solidified, and then, by taking out the cylinder block, the casting of closed deck-type cylinder block SC is obtd.