In a model-based OPC which makes a suitable mask correction for each mask pattern using an optical image intensity simulator, a mask pattern is divided into subregions and the model of optical image intensity simulation is changed according to the contents of the pattern in each subregion. When the minimum dimensions of the mask pattern are smaller than a specific threshold value set near the exposure wavelength, the region is calculated using a high-accuracy model and the other regions are calculated using a high-speed model.