A computer readable medium containing computer-executable instructions which cause a computer to execute processing steps that calculate a light intensity distribution formed on an image plane of a projection optical system. When executed, the medium causes a computer to execute the steps of dividing an effective light source into the plurality of areas, generating, for each of the plurality of areas, a plurality of shifted pupil functions by shifting a pupil function in accordance with a position of each of divided point sources, defining, for each of the plurality of areas, a matrix including the plurality of pupil functions, calculating, for each of the plurality of areas, eigenvalues and eigenfunctions by performing singular value decomposition of the matrix, and calculating, for each of the plurality of areas, the light intensity distribution based on a diffracted light distribution from the mask and the eigenvalues and the eigenfunctions.