PROBLEM TO BE SOLVED: To make it possible to obtain a diameter reduction octagonal cross section which has a cross-sectional shape which is uniform in a circumferential direction, and has a good appearance when swaging an octagonal pipe.SOLUTION: According to a method for swaging an octagonal pipe made of a metal, an octagonal pipe 2 is relatively pushed in a die 1 of which each side on an inlet side has a flat octagonal cross section, and which has an inner cylindrical surface Sof a tapered irregular shape having projections 3a, 3b, which are projected to a die center side, at a midship part of each one of eight sides of an octagon having a small diameter on an outlet side, and is diameter reduction-processed into an octagonal cross-sectional shape of an irregular shape in which the midship part of each one of eight sides of the octagon is recessed inside.SELECTED DRAWING: Figure 1【課題】八角パイプをスエージ加工する際に、周方向に均等な断面形状で良好な外観の縮径八角断面を得ることを可能にする。【解決手段】金属製の八角パイプをスエージ加工する八角パイプのスエージ加工方法であって、入口側において各辺が平坦な八角断面で、出口側において小径の八角形の８つの各辺の中央部にダイス中心側に突出する突出部３ａ、３ｂを有するテーパ異形筒状内面Ｓ２を持つダイス１に相対的に八角パイプ２を押し込んで、八角形の８つの各辺のそれぞれ中央部が内側に凹んだ異形八角断面形状に縮径加工する。【選択図】図１
