A surgery-assistance apparatus adapted to set a margin region with respect to a target region of excision in an organ including a diseased part to be removed, and the margin region being removed together with the target region, the apparatus comprising: a medical image obtainment unit adapted to obtain a medical image representing the organ including the diseased part to be removed; an organ region extraction unit adapted to extract an organ region from the obtained medical image; a structure region extraction unit adapted to extract, from the obtained medical image, a structure region representing a structure that dominates the organ and that is a tree structure; a target region determination unit adapted to specify each part of the structure which dominates each voxel in the organ region respectively, determine a dominance region in the organ as the target region, and a region in the organ other than the target region, as a non-target region, and wherein the dominance region comprises all voxels being dominated by a part of the structure, the part of which extends from a predetermined position in the structure region toward a part of the organ, the part of the organ including the diseased part; and a margin region setting unit adapted to set, based on a distance from the predetermined position (v) to each position on a boundary surface between the target region and the non-target region, the margin region to be added to the target region, characterized in that the margin region setting unit is adapted to set the margin region in such a manner that the width of the margin region gradually increases from zero to a predetermined value with increasing distance in a range in which the distance is less than or equal to a predetermined threshold value, and that the width of the margin region is the predetermined value in a range in which the distance is greater than the predetermined threshold value. a medical image obtainment unit adapted to obtain a medical image representing the organ including the diseased part to be removed; an organ region extraction unit adapted to extract an organ region from the obtained medical image; a structure region extraction unit adapted to extract, from the obtained medical image, a structure region representing a structure that dominates the organ and that is a tree structure; a target region determination unit adapted to specify each part of the structure which dominates each voxel in the organ region respectively, determine a dominance region in the organ as the target region, and a region in the organ other than the target region, as a non-target region, and wherein the dominance region comprises all voxels being dominated by a part of the structure, the part of which extends from a predetermined position in the structure region toward a part of the organ, the part of the organ including the diseased part; and a margin region setting unit adapted to set, based on a distance from the predetermined position (v) to each position on a boundary surface between the target region and the non-target region, the margin region to be added to the target region, characterized in that the margin region setting unit is adapted to set the margin region in such a manner that the width of the margin region gradually increases from zero to a predetermined value with increasing distance in a range in which the distance is less than or equal to a predetermined threshold value, and that the width of the margin region is the predetermined value in a range in which the distance is greater than the predetermined threshold value.