In a cylinder block having a water jacket, there is disposed a partition wall member which divides the water jacket into upper and lower bore sections. The partition wall member is gradually raised with increase of distance from one axial end of the cylinder block so that the volumes of the upper and lower bores sections reduce and increase, respectively, with increase of distance from the axial one end of the cylinder block. The partition wall member is formed, in the vicinity of the other axial end of the cylinder block, with openings for connecting the upper and lower bore sections. With this arrangement, the cylinder bores in the cylinder block are equally cooled by the cooling water which flows in the upper and lower bore sections, separately.