1. A method for simulating an intensity distribution of an optical image, comprising the steps of: applying a Fourier transform to the intensity distribution of the optical image formed in a resist film in a periodic direction of the intensity distribution of the optical image, and applying a spectral transform to the intensity distribution of the optical image in an aperiodic direction of the intensity distribution of the optical image by use of a base which satisfies a boundary condition; applying a Fourier transform to a modulation function for modulating the intensity distribution of the optical image, in the periodic direction of the intensity distribution of the optical image, and applying a spectral transform to the modulation function in the aperiodic direction of the intensity distribution of the optical image by use of the base which satisfies the boundary condition; computing a product of the post-transformed intensity distribution of the optical image and the post-transformed modulation function; and applying an inverse Fourier transform to the thus-found product in the periodic direction of the intensity distribution of the optical image, and applying an inverse spectral transform to the resultant product in the aperiodic direction of the intensity distribution of the optical image by use of a base corresponding to an inverse transform of the spectral transform, thereby calculating a modulated intensity distribution of the optical image.