To form a through hole at one portion of a side wall, of an expanded portion made by plastically deforming one part of a metal tube radially outward, in a portion where the plate thickness gradually changes, a slide tool is withdrawn from inside a punch hole while applying hydraulic pressure to the inside of the side wall. At the instant of commencing withdrawal of the slide tool, the degree of progress in the shearing for the portions of the side wall matching with the two side edges of the punch hole is made different. Then, the timing for moving from shearing to rupture is made to coincide, so that of the side wall, the portions matching with the punch hole, are made reliably into a blank, and are extracted from the side wall.