Abstract: The core structure of a molding apparatus for making a molded article is disclosed. The cylindrical core which forms a plurality of main core members separated by a plurality of wide slits and a plurality of complementary core members which are inserted and fastened in the slits can be independently manufactured and thereafter assembled. Accordingly, the main core members, without the complementary core plate members being attached, provide sufficiently wide machining or grinding clearance to allow speedy machining or grinding on the side faces of the main core members. The complementary core members, as well, may be machined and ground quite readily and accurately. The cylindrical core members and the complementry core members can be produced in high precision so that the assembling of the members to form a cylindrical core structure also can be carried out precisely or accurately and such core structure can be installed in the molding apparatus precisely.