PURPOSE: To prevent the occurrence of rupture, crack, etc., and to bend a flange by bending a cylindrical member to which elongation equal to that generated by bending, is previously given to the part to be bent of a flange, and making the part to be bent of the flange flat.

CONSTITUTION: A cylindrical part 11a with a rectangular section and a flange 11d erected as one body along a longitudinal direction on one side surface of the outside of the cylindrical part 11a, are provided. A cylindrical member 11 to which elongation is given so that a part 11e to be bent of the flange 11d is folded in a wave shape along the longitudinal direction of the flange 11d, is used. At a state in which both tip parts of the cylindrical member 11 are restricted by bending, the cylindrical member 11 is bent, and the part 11e to be bent is elongated. Consequently, the part 11e to be bent is elongated to become flat, there is no possibility to cause crack and rupture, which are caused by tension on the bent part 11e of the flange 11d, and bending with smaller radius of curvature can be performed to the flange 11d.