To perform the design of a roof portion of a special construction more smoothly and definitely by inputting a roof portion for a lower floor unit arranging no upper floor unit directly above on a floor plan for the upper floor and then shifting the basic design information of a roof portion input from the upper floor to the lower floor when designing the roof of a stepped unit type building.

On a first floor plan 180, input, arrangement is performed for first floor units 111, 112 and 113, and floor plan information for the first floor is prepared. Next, on the second floor plan 181, input arrangement is performed for second floor units 121, 122, also a flat roof unit 141 located directly above the first floor unit 111 is input and arranged at a hatched portion and floor plan information for second floor is prepared. Further, the location of corner portions 141A to 141D of the flat roof unit 141 is sequentially indicated and a parapet 141P is input. Thereafter, the same work is performed for the third floor and finally a roof flat 143 as a roof portion located directly above a unit 131 of the third floor is input and arranged on a third floor roof plan 183, thereby preparing roof plan information.