PURPOSE: To easily manufacture a product having odd-shaped parts, which cannot be molded integrally, by a method wherein reinforcing materials having odd-shaped part forming surfaces are fixed to the outer surface of a tub part having a U-shape in cross section and, after that, resin layers are lined starting from the interior of the recessed stepped part onto the odd-shaped part forming surfaces so as to be flush with the surface of the tub part.

CONSTITUTION: A tub part a1 is molded, at the upper end edge of which a recessed stepped part 5 opened upward and inward of the tub is provided and the cross section of which is a U-shape. The tub part a1 consists of three layers or a gelcoat layer 6, a FRP layer 7 and a backup layer 8 in the order named from the surface side. Rein forcing materials a3 are made of foamed urethane or the like, and consist of several blocks, on the surfaces of which an apron forming surface 9, a headrest forming surface 10 and an upper edge part forming surface 11 are made, and fixed through adhesive 12 to the outer surface of the tub part a1. The FRP layer 7 is lined as a first layer from the interior of the recessed stepped part 5 to the final ends of the respective forming surfaces 9, 10 and 11 and degassed and molded. The gelcoat layer 6 is lined on the hardened FRP layer 7 so as to be flush with the gelcoat layer 6 of the surface of the tub part a1 in order to be made into an integral body without producing a seam.