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, by a computer, 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, wherein the determined optical model is one of a first optical model and a second optical model, the first optical model taking into account an influence of a thickness of the mask, and the second optical model calculating a diffraction of light using a model approximately considering the pattern of the mask to be a two-dimensional object with no thickness; said plurality of optical models include high-accuracy optical models and high-speed optical models; determining the optical model is to select the high-accuracy optical model if a pattern smaller than a pattern size serving as a threshold value is included in an image intensity calculation region and select the high-speed optical model if the pattern is not included in the image intensity calculation region, and after determining the optical model and before estimating image intensity on the substrate, the method further comprises changing to the high-accuracy optical model a subregion arranged so as to enclose a subregion which has selected the high-accuracy optical model in a subregion which has selected the high-speed optical model.