A method of optimally setting exposure conditions when light emitted from an effective light source is applied to a mask pattern formed on a mask for exposure and diffracted light emitted from the mask is projected onto a substrate via a projection lens to expose the substrate thereto, the method comprising defining an image evaluation amount which represents characteristics of an optical image or a resist pattern and which contains a factor indicating the influence of a dimensional error of the mask pattern on the image characteristics, determining an initial condition of the effective light source and the mask pattern, defining at least one of a parameter of the effective light source and a parameter of the mask pattern, and changing at least one of the parameters to calculate the image evaluation amount, and deciding an optimum parameter on the basis of the result of the calculation.