Disclosed is a mask data processing method of correcting a hierarchical structure. In the case that in design data having a hierarchical structure including a plurality of cells each having a design pattern, when the total number of graphic forms or the total edge length of a design pattern on which the calculation of mask data processing is to be executed, the amount of calculation to be executed, or the expansion degree presumably becomes equal to or larger than a predetermined threshold value if the calculation of the mask data processing is executed on the design data having the initial hierarchical structure, the hierarchical structure is corrected. This correction is performed to reduce the total number of graphic forms or the total edge length of the design pattern on which the calculation is to be executed, the amount of calculation to be executed, of the expansion degree.