Abstract: An improved computer aided design (CAD) apparatus is disclosed for properly determining if a mask pattern for a large-scale integrated circuit (LSI) satisfies a predetermined design rule. This CAD apparatus includes a figure feature file for storing in advance the feature of the configuration of a contact hole region which should not be recognized as a design error. After a conventional design recognizing process has been conducted, a recognition invalidation process is effected by referring to the figure feature file. Accordingly, the detection of a pseudo-error which is not an essential error is reduced, so that the efficiency of a designing operation with this apparatus is improved.