PURPOSE: To generate corrected pattern data with an easy work by generating, such pattern data that a slit is filled at the time of the occurrence of the slit on pattern data and performing format conversion.

CONSTITUTION: Source data of a main pattern is subjected to sizing processing, and position coordinates indicating the range of an inhibited slit width and those of a pad pattern in the main pattern are compared with each other to detect whether a slit exists or not. If a slit exists, simulated peripheral pattern data generated correspondingly to the peripheral pattern and main pattern data are put one over the other, and pattern data which is so corrected that the slit is filled is generated by plus sizing processing, OR processing, and exclusive OR processing. Exposure data corresponding to the corrected main pattern is obtained by format conversion processing of pattern data and is stored on a magnetic tape MT 12. Meanwhile, data format conversion is immediately performed and obtained data is stored on an MT 11 with respect to the peripheral pattern.