Abstract: An electronic CAD system for producing layout data by using data of a predetermined pattern, which employs a novel set of data attributes for reducing the amount of layout data. The system has a layout data production processing unit for producing the following data as the layout data: identification data for a polygon used for designating an area where a plurality of the predetermined patterns are arranged; a set of vertex coordinates of the polygon; identification data of the predetermined pattern; and data for defining intervals at which the predetermined patterns are arranged. Even when patterns are arranged in an undefined area having a shape which cannot be represented by a simple array of the patterns, the array representation is performed by employing the set of vertex coordinates of a polygon for designating an area where the patterns are arranged.