There are provided a step of forming a structure in which a first concave portion is formed in a predetermined portion on one surface of a metal plate and also a convex portion that protrudes from other surface of the metal plate is formed by formation of the first concave portion and a second concave portion is formed in a predetermined portion on an outside of a peripheral portion of the convex portion, by shaping the metal plate by means of stamping using a die, and a step of cutting the convex portion formed on the other surface of the metal plate.