In this paper, two new concepts “Rooms” and “Walls” are introduced which will help to implement computer aided design systems for any type of ship. A 'Room' is a closed space made up of joined plates. The shape of a Room, described as a solid model, works as a kind of geometric constraint for plates designed in it. A 'Wall', which is a facet of a Room, represents the surface of a plate which is in the face of the Room. A Wall is used as a means for a designer to appoint an object to which parts such as bars, girders, etc. are joined. The system shown here has a few remarkable features. First, it can automatically calculate geometric models of Rooms from geometric models of plates and information of joining plates. Second, any geometric model of Room is used to calculate geometric models of plates in it. Information of joining plates is also calculated from it. In this system, all plates must belong to any one of the Rooms in a ship so as to assure that all plates have a geometric limit of its shape.