1. A region extraction system comprising: an operation unit for designating a specified point by a user; a temporary candidate region determiner for temporarily determining a candidate region of a to-be-extracted region from measured data with a region growing technique wherein the candidate region is determined by expanding an area from the specified point; an initial position determiner for determining an initial position of a standard model based on said candidate region temporarily determined; a characteristic point extractor for extracting a characteristic point of said candidate region temporarily determined; and a to-be-extracted region extractor for extracting said to-be-extracted region from said measured data with a model fitting technique using said standard model, wherein said initial position determiner determines said initial position of said standard model on the basis of the degree of matching between said characteristic point of said candidate region temporarily determined and a corresponding characteristic point in said standard model and determines said initial position of said standard model so as to obtain a minimum sum of distances each between the corresponding characteristic point after the determination of said initial position of said standard model and each of a plurality of corresponding characteristic points of said to-be-extracted region.