A pattern decomposition method capable of achieving patterns with a complicated layout by double exposure. The pattern decomposition method for decomposing a target pattern which includes first patterns having repeated lines and spaces and second patterns disposed between the first patterns and having a predetermined size into patterns for first exposure and patterns for second exposure, comprises decomposing the first patterns into a pattern for first exposure and a pattern for second exposure, decomposing the second patterns into a pattern for first exposure and a pattern for second exposure, and respectively merging the pattern for first exposure or the pattern for second exposure of the first patterns with the pattern for first exposure or the pattern for second exposure of the second patterns.