To provide a cylinder block and a manufacturing method of the cylinder block capable of easily improving rigidity of cylinder bores while securing required engine cooling performance, in the cylinder block having an open deck structure.

In the cylinder block 100, a bore 4 forming the plurality of cylinder bores 2 and a water jacket 5 as an integral gap part for enclosing the bore 4 are formed in a block body 1. In the water jacket 5, one or more piece member(s) 6 for connecting the block body 1 of an outer circumference of the water jacket 5 and the bore 4 and separating the water jacket 5 into a plurality of parts is casted. A through hole 7 for communicating the water jacket 5 separated by the piece member 6 is formed in the piece member 6.