To reduce the weight of a device, and to improve the strength, forming property and workability by using a fiber reinforced thermoplastic synthetic resin material, and bonding a reinforcing rib made of the fiber reinforced thermoplastic synthetic resin to a surface of a synthetic resin sheet.

After supplying the predetermined quantity of the fiber reinforced thermoplastic synthetic resin 4 between a male die 2 and a female die 3 having an unsealed appropriate clearance between them, both the dies are sealed, and the fiber reinforced thermoplastic synthetic resin 4 is pushed into a cavity for enlargement, and pressurized for forming, and after cooling the resin 4 for a constant time, a product is taken out of the die. As a thermoplastic synthetic resin, polypropylene and nylon 6 are desirable, and especially, polypropylene has excellent forming property and chemical resistance and it can be obtained at a relatively low cost. As a reinforcing fiber, glass fiber and carbon fiber are desirably used. The glass fiber having short fiber is desirable, and included at 20-40 wt.% in relation to the whole of the fiber reinforced thermoplastic synthetic resin.