A CAD apparatus includes an input device, a central processing unit, and a display device. In the CAD apparatus, the central processing unit uses an intersection/contact point calculation function to determine diagrammatical elements of points, lines, and circles which are additional elements by using the points, the lines, and the circles as fundamental elements in plotting and inputting necessary parameters to the input device, so that a macro-diagram is formed on a screen of the display device, with a diagram-forming process being performed by accumulation of constraint conditions.