PURPOSE: To prevent oil from leaking from an oil gallery to a cylinder block outside by providing a distributing passage on a crank case metal mold, cast punch-forming by a pin shaped jig and a supporting jig, and installing an oil jet on the distributing passage.

CONSTITUTION: A cast-punching hole by which an oil gallery 7 extending in crank shaft direction is formed, is formed by a pin shaped jig 17 for forming the cast-punching hole, and an oil passage is provided to communicate from the cast-punching hole to an oil jet 13 through a distributing passage 11. The distributing passage 11 is provided in a crank case metal mold for forming the inner wall of a crank case. The pin shaped jig 17 is cast-punching-formed by a supporting jig 18 for supporting at least one position. The oil jet 3 is installed to the distributing passage 11 to inject oil inside a cylinder 6. It is thus possible to form a cylinder block which allows no oil leakage from the oil gallery 7 to the outside of the cylinder block.