PURPOSE: To provide a CAD line width data compatibility system by which the correspondence table of line width identifiers on a self system and an opposite party system is previously generated and drawing exchange can be performed in considerably smooth and efficient.

CONSTITUTION: Corresponding data on the line width identifiers of the self system and the opposite party system are previously generated and are stored in storage means 400, 500 and 600. The data expression form of the opposite party system is stored in a storage means 300. At the time of exchanging a drawing, form corresponding means 3 or 4 reads corresponding data of the line width identifier in correspondence with the expression form obtained by an expression form judgement means 2 from an opposite party system name. Then, it is referred to and a line width data conversion means 5 converts data so as to mutually exchange data.