PURPOSE: To maintain uniformly the strength of welt and to prevent deterioration of the appearance characteristic thereof by slitting a belt-like metal and stretching the metal in the longitudinal direction thereby forming the metal into the shape in which intermediate regions are longitudinally connected.

CONSTITUTION: A belt-like metal 21 is formed by a mechanical working means and thereafter the metal is rolled in the longitudinal direction thereof by a press, etc. around the intermediate region 26 in the central part to form thinly the region 26. The two intermediate regions 27, 28 positioned on both end sides in the transverse direction of the metal 21 are similarly pressed down to an extremely small thickness by rolling means from upper and lower directions and are thereafter cut to pieces. The blank material formed in the above-mentioned way is bent at the two points in the suitable position around the region 26 and a core material 29 is completed.