1. A system comprising: a memory configured to store medical image data; an aneurysm identification device which identifies an aneurysm in the medical image data; a distortion-degree evaluation device which quantitatively evaluates a distortion degree of the aneurysm; and a rupture risk derivation device which derives a rupture risk of the aneurysm from a result of the evaluation; wherein the distortion-degree evaluation device includes: an approximation-by-ellipsoid device which approximates a shape of the aneurysm by a shape of an ellipsoid; and a difference evaluation device which performs a subtraction between the shape of aneurysm and the shape of the ellipsoid and evaluates a thickness at each surface position of a difference between the aneurysm and the ellipsoid as the distortion degree.