An axial cast hole is formed to have a large diameter side end on one side at a draft angle. A large diameter hole is formed on the one side to form a stepped surface between the large diameter hole and the axial cast hole. The stepped surface has an inner periphery end located on a plane perpendicular to the axial direction. A tool is moved from the one side toward the other side coaxially with the axial cast hole to make contact with the stepped surface. The tool is moved subsequently to cut the inner periphery of the axial cast hole between the large diameter hole and the first port. The tool is moved subsequently to cut the inner periphery of the axial cast hole on the other side relative to the first port.