1. A method of correcting a mask pattern which is designed to form a semiconductor integrated circuit and is transferred onto a substrate by light exposure, the method comprising: dividing the mask pattern into a plurality of subregions; determining an optical model to estimate image intensity formed on the substrate from a plurality of optical models on the basis of the relationship between the degree of fineness of the patterns included in each of the subregions and the wavelength of exposure light; and estimating image intensity on the substrate for each of said plurality of subregions using the determined optical model.