PURPOSE: To effectively perform the interior design of an architectural design CAD by setting the distance between the exterior surface formed to a structure wall and the surface of the counter wall of the exterior wall at an integer multiple of a prescribed standard unit.

CONSTITUTION: The walls X1, X2, Y1 and Y2 are defined as the standard X and Y direction structure walls. When the distances between the cores of both structure walls and an inner room surface are different, an exterior wall is set at a position apart from one of both structure wall surfaces by 60mm. For instance, a grid is decided when the exterior walls are set at the positions apart from the surfaces of walls X1 and Y1 by 60mm. Then the exterior positions of the walls X2 and Y2 are also unconditionally decided within a range of 60-85mm. Furthermore the exterior wall positions are automatically calculated so that the space between these counter exterior walls is equal to a standard module set previously. thus it is possible to perform the effective interior design of a group housing, etc., by a CAD system and also to promote the standardization of various interior parts.