A method which determines patterns for a plurality of masks to be executed by a processor includes 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 an angle of a line connecting a plurality of pattern elements in each mask. In the method, a pattern of each mask is determined based on the calculated evaluation value.