PURPOSE: To shorten a process by mounting an annular rim stock between an upper ring and a lower ring which are concentric to each other and relatively movable in the axial direction and approaching both rings relatively.

CONSTITUTION: The rim stock is set on the lower ring 16, an upper ram 10 is lowered, the upper ring 14 is lowered, the taper (a) of the tip part of the upper ring 14 is inserted into a bead seat part (b) continued by the flange of a work and pressurized and the bead seat part (b) is expanded forcedly by the taper (a) outward in the radial direction. Simultaneously, the work is pressed downward by the upper ring 14 and the gutter part (c) of the work is contracted by the taper part (d) of the lower ring 16 inward in the radial direction. In this way, the expansions of the bead seat part (b) and the flange part of the work and the contraction of the gutter part (c) are performed simultaneously. When the upper ring 14 presses the work further downward and the bead seat part (b), the flange part are expanded further in the radial direction, the step part of the upper ring 14 is inserted into the internal peripheral side of the flange part of the work to be formed into the final shape.