1. A method for determining patterns for a plurality of masks used for a mask manufacturing method for manufacturing the plurality of masks, the method being executed by a processor, comprising: acquiring data on a pattern containing a plurality of pattern elements; and assigning the acquired plurality of pattern elements into masks, decomposing the acquired plurality of pattern elements into patterns of the masks, and calculating an evaluation value for an evaluation index, based on a number of masks, the distances between a plurality of pattern elements in each mask, and angles of lines connecting a plurality of pattern elements in each mask, wherein the evaluation index has a term defined by difference between the angles and a first predetermined angle in one mask illuminated by a first illumination and a term defined by difference between the angles and a second predetermined angle in other mask illuminated by a second illumination different from the first illumination, wherein the first predetermined angle is different from the second predetermined angle, wherein a pattern of each mask is determined based on the calculated evaluation value, and wherein the mask manufacturing method comprises a step of manufacturing the plurality of masks, each mask including the determined pattern.