1. A method of arranging mask patterns, the method comprising: separately calculating contributions of an assist feature to image intensity at a main feature at a first focus position and at a second focus position by using a computer; placing the assist feature at the second locus position when the contribution of the assist feature to the image intensity at the main feature is greater at the second focus position than at the first focus position; obtaining a first contribution function in a first operation for determining the contribution at the first focus position of an assist feature to image intensity at a main feature; obtaining a second contribution function in a second operation for determining the contribution at the second focus position of the assist feature to the image intensity at the main feature; determining a position of the assist feature in a third operation that satisfies a condition in which a linear combination of the first contribution function and the second contribution function exceeds a predetermined threshold value; and arranging the assist feature within the mask patterns at the position satisfying the condition.