The present invention provides a decision method which decides a mask pattern used in an exposure apparatus comprising a projection optical system that projects a mask pattern including a main pattern and an auxiliary pattern onto a substrate, and an exposure condition in the exposure apparatus, the method including a step of calculating an image of a mask pattern formed on the substrate by the projection optical system while changing settings of the mask pattern and the exposure condition, and deciding the mask pattern and the exposure condition based on the image of the mask pattern, wherein the step includes determining whether or not to generate a new auxiliary pattern after the settings are changed.