PURPOSE: To provide sufficient rigidity in a rear wall of a cabin, simplify a structure, and facilitate manufacture.

CONSTITUTION: A hull member 1 having a ship's side plate and a ship's bottom plate, a deck member 2 having an upper structure 24 of a cabin and a deck and a hull liner 4 having a lower structure of the cabin, are respectively composed of a plastic integral mold, and the hull liner 4 and the hull member 1 are mutually joined together through a reinforcing member 51 or the like, and the upper structure 24 of the cabin and the hull liner 4 are butted on each other, and are integrally joined together. Therefore, a deck floor 21 of a cockpit is formed on the rear side of an upper rear wall 25 of the cabin, and a bull work 23 is formed on both sides of this cockpit, and a doorway to go in and out the cabin from the cockpit is formed in this upper rear wall 25, and the upper edge of a lower rear wall 42 of the cabin is joined to a bulkhead 51 arranged on the ship's bottom plate of the hull member and a front end part of the deck floor 21 of the cockpit of the deck member.