To provide a cylinder block for an internal combustion engine, capable of reducing LOC by reducing bore deformation while securing a cooling water passage.

On the lower side sufficiently distant from the upper part of the cylinder block 1, a female portion 31 is formed to which the front end of a head bolt 5 is threaded for mounting a cylinder head. On the upper side of the female portion 31 between the upper part of the cylinder block and the female portion 31, the cooling water passage 2 is formed which has a large cross section area and through which the head bolt 5 passes.