1. A medical instrument, comprising: a storage portion that stores three-dimensional image data of inside a body of an examinee that is previously acquired; an organ extracting portion that extracts a predetermined organ from the three-dimensional image data; a first region designating portion that designates a first region of the organ in the three-dimensional image data; a blood vessel extracting portion that extracts a plurality of blood vessels that are inside the organ from the three-dimensional image data; a blood vessel classifying portion that classifies the plurality of blood vessels that the blood vessel extracting portion extracts into either arteries or veins; a template creating portion that creates a blood vessel region template comprising a blood vessel-related region in which a diameter of at least either one of the arteries and the veins classified by the blood vessel classifying portion is increased by a predetermined amount; and a second region extracting portion that, based on the blood vessel region template, extracts a second region that is a region that is classified according to anatomical features of the organ and that includes the first region.