To efficiently conduct pre-hemming of a flange around a notched part of a panel regardless of its shape.

Plural slide bars 27, which are arranged in the circumferential direction of an unit main body 25 of a pre-blade unit 13, are slidably supported to the unit main body, plural pre-blade segments 28 constituting a preblade 12 are mounted to a tip of each slide bar 27, each slide bar 27 is constantly energized with a spring, the pre-blade segments 28 are converged around a pr-eblade drive shaft 30 in center, the pre-blades 12 are accommodated inside a forming die 11. By oscillating a first crank bar 37 with a drive cam 48 arranged to an upper die 14, the pre-blade 12 is moved to a two point chain line position above the forming die 11 through a lower slide cam 19, next, by oscillating a second crank arm 42, each slide bar 27 is retreated through a pre-blade drive shaft 30, the pre-blade segment 28 is descended while advancing in the radial direction, prehemming is done, successively, hemming is done by a hem blade 15 arranged to a upper die 14.