PURPOSE: To prepare each of a basic plan, an each ground floor plan, and an each floor roof construction plan by preliminarily storing each kind of control programs necessary for preparing plans, and following the control programs stored in a storage means by using inputted data and parameters.

CONSTITUTION: Data concerning the floor plan and the roof plan of a building are inputted, and simultaneously the automatic preparing parameters for automatically preparing the plans are inputted. In this case, the inputs of these floor plan and roof plan are operated while inputting necessary coordinate data from a keyboard 8, and plotting them on a display means 9. Thus, the basic plans are prepared and this result is displayed at the display device 9.