PURPOSE: In a double-skin type ship, to facilitate the provision of a draining port and to reinforce the side of the hull by making the side wall of the floor protrude outward and attaching the protruding part to the outer shell of the ship and forming the drain port on the protruding part.

CONSTITUTION: Both sides of the floor 2 are raised to form bottom side walls 4. The upper side of each bottom side wall 4 is bent substantially horizontally to form a stepped part 5. The outer side of the stepped part 5 is raised to form an upper side wall 7. The upper side of the upper side wall 7 is bent horizontally and the edge part thereof is overlapped with and fastened to the upper edge part of the outer shell 1 by rivets or the like together with a fender 29. A protruding part 40 protruding outward is formed on the side wall. A drain port is formed on the protruding part 40 at the same level with the floor. The lower part of the protruding part 40 is in contact with and is made to adhere to the outer shell 1.