A cylinder block (10) includes an inter-bore flow passage (14) between adjacent bores (12). Each inter-bore flow passage (14) is formed by closing an opening-side groove portion (15) continuous from a cylinder block deck face and a bottom-side groove portion (16) with the use of a lid member (20) made of copper, and the lid member (20 is laser-welded to a groove wall face of the opening-side groove portion (15). Before the laser-welding, the lid member (20) has recesses (26) that are interspersed in a direction, in which a laser beam travels, at any one of connected portions at which the lid member (20 is connected to the groove wall face of the opening-side groove portion (15).