PURPOSE: To hold an internal cabin in the horizontal position despite the rock of a hull by supporting the internal cabin in rockable manner on the inner side of the hull through rollers on the side face and rollers on the bottom part.

CONSTITUTION: Rollers 3 corresponding to rolling are installed on the internal side of a hull 1, and at the bottom center part on the internal side of the hull 1, rollers 4 are partially disposed in the curved face state. In an internal cabin 2 with the side face plate 6 formed into semi-circular curved face is supported on the rollers 4. A weight chamber 5 serving as a weight is provided at the lower part of the internal cabin 2 so as to accommodate fuel, liquid, and the like therein and also to hold the internal cabin 2 in the constantly horizontal state against the rock of the hull 1.