To improve impact resistance from the outside, reduce weight and easily perform maintenance by continously and integrally forming hat stiffener outside plating parts projecting to the inside at prescribed portions of an outside plating made of FRP and providing hat stiffener heartwood within the hat stiffener outside plating part.

The hull 2 of a vessel 1 made of FRP is constituted of a bottom of vessel outside plating 3 made of FRP, a vessel side outside plating 4 made of FRP and an upper deck made of FRP. The bottom of vessel outside plating 3 made of FRP continously and integrally forms hat stiffener outside plating parts 3A projecting to the inside at a plurality of prescribed portions at the time of laminating and forming the bottom of vessel outside plating 3 made of FRP. The hat stiffener 3A has a recessed portion opened to the outside and is integrally manufactured by performing primary adhesion after a synthetic resin material 7 is mounted within the recessed portion. Thus, pressing force concentration can be evaded, workability can be improved and quality can be improved.