PURPOSE:To prepare and edit mask data which do not cause an error on circuit expression by displaying a mask pattern based on the mask data and a circuit pattern data based on circuit data relatedly on the same screen. CONSTITUTION:Mask patterns 1 and 2 of wiring and wirings 4 and 5 on the circuit data corresponding to them are displayed relatedly on the same screen. Then, since the wirings 1 and 2 are erroneously connected by a mask pattern 3 of erroneous contact, a pattern 6 to show the erroneous connection is displayed with a dotted line in a circuit pattern display part and the erroneously connected wiring parts 1, 2, 3 and 4 are flashingly displayed. Thus, the occurrence of the error of the preparing and editing of the mask data are its place can be confirmed.
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