PURPOSE: To provide an exposure data processing method of a mask ROM integrated circuit wherein a time for exposure data preparation is reduced.

CONSTITUTION: This method is an exposure data preparation processing method of a mask ROM integrated circuit wherein only a pattern of a ROM part for storing specified information according to write data by a single kind is different and a pattern excepting the ROM part is the same. A cell formation layer for generating a pattern stored in the ROM part is divided into a plurality of small regions of a specified size. ROM pattern generation processing in each small region is carried out respectively by parallel processing by a plurality of processing means.