PURPOSE: To facilitate the manufacturing of a hull in which openings are provided in the side walls of a cockpit to utilize its space, by making the opening etc. when forming a hull member by a mold.

CONSTITUTION: In the opening 5 of each side wall of a cockpit, a wall portion 4 erected from a floor 2 is bent outwards so that a horizontal part 7 is provided and its outer end is butted to the wall portion 11 of the outside plate of a hull and conjoined to the wall portion by a sealing material. To manufacture the side wall of the cockpit, a core 90 corresponding to the opening 5 is secured on a mold 9 by bolts 90 and fiber-reinfored plastic is placed on the mold 9. Fiber-reinforded plastic is also placed on the bottom of the core 90 to form the horizontal part 7. After the moldig work, the core 90 is removed and the molded product is seaparated from the mold 9. A hull member is thus provided. The hull member is conjoined to the wall portion 11 of the outside plate of the hull.