A dominance region in an organ (5) dominated by a part (61) of a structure is determined as a target-region (51), and a region in the organ (5) other than the target-region (51) is determined as a non-target-region (53) . Thepart (61) extends from predetermined position (v) in a structure region toward a part of the organ (5) including a diseased part. A margin-region (52) is set based on a distance from predetermined position (v) to each position on a boundary surface between the target-region (51) and the non-target-region (53). The width of the margin-region (52) 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 (52) is substantially constant in a range in which the distance is greater than the predetermined threshold value.
