1. A region extracting method for a medical image, in which a region of a predetermined organic part is extracted from an image captured for medical use, said method comprising the steps of: determining from data of a plurality of images in which substantially the same region of said organic part is captured under measurement conditions different from each other measurement values concerning predetermined characteristics set for respective images concerning respective pixels located at positions corresponding to each other between said images; determining, in a multidimensional characteristic space defined by a plurality of coordinates indicating respective measurement values of said images, a sample distribution of said pixels relating to said region of said organic part employing said respective measurement values of said characteristics as coordinate values, and applying a distribution function taking account of a correlation between said respective characteristics of said images to said sample distribution so as to estimate a matrix distribution range corresponding to a predetermined tissue region belonging to said organic part; and extracting an assembled area of said pixels included in said matrix distribution range in each of said images as a region corresponding to said tissue region.