PROBLEM TO BE SOLVED: To unfailingly perform pre-hemming even when an upper mold is moved down at a high speed.SOLUTION: A hemming processing device includes: a lower mold 200a; an upper mold 200b which may move up and down; a pre-hem blade 221 which is provided at the lower mold 200a and may slide in a horizontal direction; a cam driver 223 provided at the upper mold 200b; and a cam mechanism which is pressed downward by the cam driver 223 thereby sliding the pre-hem blade 221 to a workpiece. Flat surfaces are provided on a lower surface 223d of the cam driver 223 and an upper surface 225a of an upper cam 225 of the cam mechanism 222, which contact with each other, in a horizontal direction.
