PURPOSE: To improve an antislipping action of a surface on the die surface side of a plate material, and facilitate manufacture in the plate material such as an FRP deck member by die forming.

CONSTITUTION: In a deck 11 integrally molded so that an upper surface of a product becomes the die surface side of a forming die in die forming of FRP, a large number of projections 2 are formed almost uniformly distributively on the upper surface, and in these projections 2, a cross-sectional shape is an almost trapezoid, and a recessed part 23 is formed in a central part of the upper surface, and an inclination (α) of the inclined surfaces 21 in the outer peripheral parts of the projections 2 is formed in the size so that a trimming gradient is secured even in an inclined part of the deck 11, and a depth h2 of this recessed part 23 is formed smaller than a height h1 of the projections 2.