A streak image enhancing unit subjects an ultrasound image representing a blood vessel to processing using a two-dimensional filter that is longer in the long axis direction of the blood vessel than in the short axis direction thereof so as to enhance a streak image corresponding to the long axis direction. A trace line candidate extracting unit extracts from the ultrasound image with the enhanced streak image a plurality of trace line candidates on the basis of the output values of the two-dimensional filter. A trace line selecting unit selects two trace lines corresponding to the inner boundary of the tunica intima and the boundary between the tunica media and the tunica externa of the blood vessel out of the plurality of trace line candidates on the basis of an evaluation value relating to continuity acquired for each trace candidate.