A dominance region in an organ dominated by a part of a structure is determined as a target region, and a region in the organ other than the target region is determined as a non-target region. The part of the structure extends from predetermined position v in a structure region toward a part of the organ, the part including a diseased part. Further, a margin region is set based on a distance from predetermined position v to each position on a boundary surface between the target region and the non-target region. The width of the margin region gradually increases from zero to a predetermined value in a range in which the distance is less than or equal to a predetermined threshold value, and the width of the margin region is substantially constant in a range in which the distance is greater than the predetermined threshold value.