To provide a cylinder block manufacturing method for manufacturing a closed deck type cylinder block with a bridge and a water jacket shaped as desired, while improving the quality and productivity.

The manufacturing method is provided for the closed deck type cylinder block 1 which has a water jacket 10 formed to encircle a plurality of cylinder liners 11 arranged side by side, and a top deck having a bridge portion 6 for covering part of the upper end opening of the water jacket 10. First, a water jacket core 20 formed of a zinc alloy with a coat film of 2-10 m on the surface is set on a mold. Next, aluminum alloy molten metal is filled in a cavity to cast the cylinder block 1. Next, heat insulating demolding agent is applied to the side of the top deck of the cylinder block 1 in the state that the water jacket core 20 remains. Then, the cylinder block 1 is heated to remove the water jacket core 20.