To provide a core capable of forming a notch for adjusting the head rest height even in a pipe member consisting of an electric welded tube with a weld bead present therein, and consistently press-forming the notch to a desired groove shape by a simple groove structure.

The core for notch press forming of a head rest stay forms a contour shape of the entire length in the axial direction while a narrow arc-shaped supporting face 1 of the same diameter as the inside diameter of a tubular member P forms a top part, both notched faces 21and 22inclined from both ends 1a and 1b of the top part 1 form run-off parts while maintaining angles θ1and θ2in the outward of the perpendiculars 11and 12with both ends 1a and 1b of the top part 1 as a standard by 30°, and a wide arc-shaped supporting face 3 of the same diameter as the inside diameter R of the tubular member P on a remaining face having the top part 1 and the run-off parts 21and 22forms a bottom part. An arc-shaped face of the radius r2subtracting the projecting dimension r1of a recessed part X from the radius of the tubular member P forms a bottom face 4a leading to both notched faces 21and 22. Recessed grooves 41-44are formed in the top part 1.

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