PURPOSE: To prevent breakage of a core and to facilitate casting work by forming outer wall at top part of a cylinder block, arranging a movable die having comb teeth-like projecting plate part and fitting the assembling core into gap of the projecting plate part at the time of assembling the core.

CONSTITUTION: Plural projecting plate parts 132 inserting in space facing to a water jacket are formed as the comb teeth-like at the molding board part in an upper die 13, and a pressure-resistant sand core 33 as the assembling core is inserted into the gap and arranged. A liner 35 fitting the sand core 36 is fitted to the fixed position in space part 111 facing to a crank case on a fixed die 11. After assembling the core, die clamping is executed with a side cylinder 16, upper cylinder 15 and front and rear cylinders, respectively, and then, casting stage is executed. As the sand core 33 is firmly supported through the projecting plate parts 132, the breakage is prevented to unbalanced load. As the core 33 can be remained at the time of opening the die, die casting can be operated and the casting work is facilitated.