1. A non-transitory computer-readable storage medium storing a program that causes a computer to decide an exposure condition in an exposure apparatus including an illumination optical system for illuminating a mask, and a projection optical system for projecting an image of a pattern of the mask onto a substrate, the program causing the computer to execute: a first step of selecting an evaluation item of interest that is an evaluation item from a plurality of evaluation items calculated from an image formed on an image plane of the projection optical system in correspondence with the exposure condition for evaluating the image in comparison with a target pattern to be formed on the substrate; a second step of selecting an auxiliary evaluation item from the plurality of evaluation items which is different from the evaluation item of interest and changes a value in the same direction as that of a change in a value of the evaluation item of interest upon changing a parameter value of the exposure condition; a third step of setting an evaluation function including the evaluation item of interest and the auxiliary evaluation item as values; a fourth step of changing the parameter value of the exposure condition so as to make a value of the evaluation function closer to a target value and calculating the value of the evaluation function for each of a plurality of images formed on the image plane of the projection optical system in correspondence with the changed parameter value of the exposure condition; and a fifth step of deciding the parameter value of the exposure condition corresponding to a value of the evaluation function satisfying the target value out of the plurality of values of the evaluation function calculated in the fourth step.