A plurality of protruding portions is formed in a metal plate material through a plurality of steps by use of forming tools (23, 24) each of which includes a die and a punch (26, 29). An arrangement pitch (p2) between a concave portion (292) and a convex portion (291) for use in forming the protruding portions in the die and the punch (29) that are used in a post-process is made narrower than an arrangement pitch (p1) between a concave portion (262) and a convex portion (261) for use in forming the protruding portions in the die and the punch (26) that are used in a pre-process.