Abstract: Forming a structure of seat back with armrest is effected by pressingly adhering a covering material to a foam padding of a predetermined sheet back shape, through a foaming process and an adhering-type stamping process, using a die element having a protuberant die surface part in both of the foaming and adhering-type stamping processes. The protuberant die surface part includes at least two sloped die surfaces. Hence, in operation, the die element can be smoothly removed from a resulting foam padding in which a recessed portion with at least two sloped walls is formed, at the foaming process. Then, the covering material is bonded evenly in such recessed portion and other surrounding area of padding at the adhering-type stamping process.