Cylinder block of internal combustion engine

A cylinder block of an internal combustion engine, the cylinder block includes a partition wall arranged between a plurality of cylinder bores, the cylinder bores are adjacent to one another; and a coolant passage that is arranged in the partition wall. The coolant passage includes (i) a large diameter hole not intersecting a virtual plane, the virtual plane includes central axes of the plurality of cylinder bores and (ii) a plurality of small diameter holes each having a diameter that is smaller than a diameter of the large diameter hole, the plurality of small diameter holes communicate with the large diameter hole, and the plurality of small diameter holes extend in a different direction than a direction that the large diameter hole extends, the plurality of small diameter holes intersect the virtual plane.