To expand a polygonal shape into a plate having excellent flatness and wide width by successively expanding the polygonal shape having a slit at a cross sectional shape and discontinuous plural bend parts with a bend part as a center and expanding the bend part furtherest apart from the slit at last thereby preventing a plate part from largely bulging/deforming.

A metal hexgonal shape 1, which is made discontinuous by slits S and in which plate parts 1a-1f are joined through more than three each, for example five bend part 1g-1k, is successively expanded from the slits S with the bend parts 1g-1k as a center to obtain a wide width plate P. In the expanding method, respective vacuum adsorption bodies 3a, 3b are applied to one of plate parts 1a, 1b adjacent to the slit S, the plate part 1a is expanded around a first bend part 1g, next, the plate part 1b is expanded at the bend part 1h. Subsequently, other plate parts 1f, 1e are successively expanded, at the end, the plate parts 1c, 1d are expanded at the bend part 1i furtherest apart form the slit S.