1. A method for generating a pattern of a mask used for an exposure apparatus that exposes a substrate with light by using a projection optical system that projects an image of the pattern of the mask on the substrate through a calculation by a processor, the method comprising the steps executed by the processor: applying, to a target main pattern, a reference map of a characteristic value of an image of a representative main pattern with respect to a position of a representative auxiliary pattern calculated for each of a plurality of positions while the position of the representative auxiliary pattern with respect to the representative main pattern is changed and calculating a map of the characteristic value of the image of the target main pattern with respect to a position of an auxiliary pattern; and determining the position of the auxiliary pattern by using data of the map of the characteristic value of the image of the target main pattern and generating a pattern of a mask including the target main pattern and the determined auxiliary pattern.