1. A non-transitory computer readable storage medium storing a computer-executable program for causing a computer to determine at least one of mask pattern and exposure condition of an exposure apparatus having an illumination optical system for illuminating a mask with light from a light source and a projection optical system for projecting an image of the mask pattern onto a substrate, wherein the program causes the computer to perform the following steps: setting a parameter about at least one of the mask pattern and the exposure condition; setting information to define an image blur of the mask pattern formed at a single point on the substrate so as to superimpose a plurality of laterally shifted images of the mask pattern upon one another at the single point by illuminating the mask and projecting the laterally shifted image of the mask pattern onto the single point on the substrate with scanning the mask and the substrate; calculating the image of the mask pattern at the single point to be formed of the superimposed laterally-shifted images by projecting the laterally shifted image of the mask pattern onto the single point on the substrate with scanning the mask and the substrate under the exposure condition using the set parameter and the set information; changing the set parameter; calculating the image of the mask pattern at the single point to be formed of the superimposed laterally-shifted images using the changed parameter and the set information, after changing the set parameter; repeating the step of changing the parameter and the step of calculating the image of the mask pattern to be formed of the superimposed laterally-shifted images using the changed parameter and the set information; and determining at least one of the mask pattern and the exposure condition based on calculation results of the images obtained by repeating the step of changing the parameter and the step of calculating the image.