PURPOSE: To secure a sufficient sealing property by coupling the inner end section of a deck reinforcing piece inserted between inner and outer wall sections at the open end section of the water jacket of a cylinder block with a step section provided on an inner wall section, and welding an outer end section to an outer wall section.

CONSTITUTION: A trough-shaped water jacket 12 opened at the end section on a deck side is formed around a cylinder bore 14 in an open deck type aluminum cylinder block 11, and a deck reinforcing piece 13 is inserted between inner and outer wall sections 11b, 11d of the open end section of the water jacket 12 and welded and fixed there. A recess 11d as a step section is formed at the open end portion on the outer periphery of the inner wall section 11b of the water jacket 12. When the deck reinforcing piece 13 is inserted, the inner end section of the deck reinforcing piece 13 is coupled with the recess 11d, and the outer end section is fixed to the outer wall section 11d of the water jacket 12 with a welded section 16 melted and formed by a high-energy beam such as an electron beam.