PURPOSE: To increase rigidity of a cylinder block by integrately arranging seat metals on inner and outer circumferences of upper end opening part, covering metal-made cover plate having window for water passage of outer shape coinciding with the opening part, on the opening part and joining to the seat metal.

CONSTITUTION: In the casting process of the cylinder block 1, the seat metals 9, 10 are inserted in the die and is cast-in at the inner and outer circumferences of the upper end opening part of a water jacket 4. The seat metals 9, 10 is made of steel plate and each brim-like part bending to L-shape is faced to the water jacket 4 side and this brim-like part is made to the same face as the seat metal 9, 10. The steel plate-made cover plate 14 is covered on the upper end part of the water jacket 4 in the top deck 7 face. The cover plate 14 is fitted into the L-shape part of seat metal 9, 10 and joined by welding, etc. In this result, the upper end part of outer circumferential wall 3 of the cylinder becomes construction connecting with the outer circumferential wall 5 of the water jacket through the cover plate 14, and the rigidity is improved and as the production method is simple, high productivity is obtd.