Dental information is useful for personal identification. In this paper, a method for extracting three-dimensional shape of a tooth automatically from dental CT images is proposed. In the previous method, one of the main issue is the mis-extraction of the adjacent region caused by the similarity of feature between a tooth and its adjacent teeth or the surrounding alveolar bone. It is important to extract an accurate shape of the target tooth as an independent part from the adjacent region. In the proposed method, after denoising, the target tooth is segmented to parts such as a shaft of a tooth or a dental enamel by the mean shift clustering. Then, some segments in the certain tooth is extracted as a certain region by the region growing method. Finally, the contour of the tooth is specified by applying the active contour method, and the shape of the tooth is extracted.