PURPOSE: To efficiently manufacture a material of which the width size changes in its longitudinal direction from a long material of which the same sections continue in its longitudinal direction by cutting the long material diagonally with respect to the longitudinal direction to form a pair of blank having angles of an approximately triangular shape and joining these.

CONSTITUTION: A long material 1 is cut from one side end face up to the midpart in its longitudinal direction along cutting lines 2, 3 inclined with respect to the axial line N-N in the longitudinal direction of the material 1, whereby a wall 4 of a triangular shape shown by diagonal lines is cut off. The material 1 is likewise cut from the other side end face of the material 1 up to the midpart in the longitudinal direction along cutting lines 5, 6 inclined with respect to the axial line N-N in the longitudinal direction, whereby a wall 7 is cut off into a triangular shape. Blank materials 10-1, 11-1 and 10-2, 11-2 which oppose above and below and assume a pair are pressed in the mutually approaching directions and are bent from the root parts of coupling, thence obliquely cut faces 10-1a, and 11-1a as well as 10-2a and 11-2a are joined by welding, and rear side cut faces 10-1b, 11-1b, 10-2b, 11-2b are welded to the side faces of a remaining wall 12.