PURPOSE: To dispense with welding and simplify manufacturing process of a core holder by anchoring a sliding plate by means of louvered pieces to the outer periphery of a cylindrical member formed by drawing a galvanmized iron sheet.

CONSTITUTION: A core holder 16 is a cylindrical member formed by drawing a galvanized iron sheet and which has Z-shaped hook parts 5 and 6 formed by louvering. In case of anchoring a sliding plate 1 to the cylindrical member, first, one anchor piece 9 of the sliding plate 1 is located in the hook parts 5 and another anchor piece 9 is located in the hook parts 6 which are not subjected to bending work yet, and then bending work is made. Consequently, welding work can be dispensed with in the process from the formation of a core holder 16 to the fitting of the sliding plate 1. Therefore, it becomes unnecessary to carry out galvanizing for rust proofing which is otherwise required after welding, and it becomes possible to use from the outset a galvanized iron sheet to which rust prevention has been applied. Therefore, not only the manufacturing process can be simplified, but also the deformation of the core holder which thends to occur during spot welding can be eliminated.