A method of simulating an optical intensity distribution on a substrate when a mask pattern formed on the mask is transferred to the substrate through a projection optical system by irradiating an illumination light obliquely on a mask surface of the mask, which comprises setting a phase difference between a zero-order diffraction light and a first-order diffraction light determined according to at least one of a distance between the zero-order diffraction light and the first-order diffraction light on a pupil of the projection optical system, thickness of a light-shielding portion formed on the mask, angle defined by an optical axis direction of the illumination light and an incident direction on the mask, and a difference between a size of the mask pattern and a half cycle of the mask pattern, and carrying out a simulation of the optical intensity distribution on the substrate according to the set phase difference.