To provide a forming technique which enables elimination of a burr on the occasion of forming a through hole.

A gap is provided beforehand between the butting faces of the first and second mold members, allowing for a difference between the amount of thermal expansion of the first and second mold members and that of the rod-like core material at normal temperatures, in a state wherein the fore end face of a rod-like core material fitted to a first mold member is brought into contact with the bottom part of a molding material filling recess of a second mold member. In this constitution, the gap becomes zero due to the amount of thermal expansion at a raised temperature and thus the first and second butting faces are brought into contact with each other.