Cylinder block of multi-cylinder engine and process of molding same

A process of molding a cylinder block of a multi-cylinder engine comprises making a jacket core (30) so as to form cylinder jackets (8) of the multi-cylinder engine (E), attaching the jacket core (30) to a cylinder block forming mold (28), and pouring molten metal into the cylinder block forming mold (28). The process makes a water passage forming core (31) of sphered particle sand having a lower expansion coefficient than the common silica sand. The core (31) is intended for forming at a head side portion of an inter-bore wall (4) of the multi-cylinder engine (E), a cooling water passage (10) which communicates the cylinder jackets (8) with a head jacket (22). And prior to pouring the molten metal, the process fixedly attaches the water passage forming core (31) to a position corresponding to the inter-bore wall (4) of the jacket core (30).