Provided is a mask data processor capable of expanding a design data throughout the entire area of a mask. A storage device (MR) inputs a design data of sub-chips (D1) and mask data creation specification data (D2) to a pattern-density data generation device (10). Then, a data execution part (11) performs an arithmetic execution to the design data of sub-chips (D1) based on the mask data creation specification data (D2), followed by automatic mask data generation processing, layer arithmetic execution processing, and dummy pattern generation processing. When calculating a pattern graphic area, a graphic area calculation part (12) eliminates any overlap between graphics in order to avoid duplicate calculation. Based on the pattern graphic area, a pattern-density data calculation part (13) calculates the area ratios of graphic elements, i.e., pattern elements, contained in a unit region.