A first interface of a media-adventitia interface and a second interface of a lumen-intima interface are specified on the basis of reflected waveform data in an n-th frame, a first interface of a media-adventitia interface and second interfaces of a lumen-intima interface are specified on the basis of reflected waveform data in an m-th frame, a first interface displacement amount and second interface displacement amounts between the n-th and m-th frames are determined, a reference displacement amount is calculated from the same reflected waveform data by phase difference tracking as the displacement amount of a vascular wall, and evaluation values for evaluating interface specification credibility are calculated from the first interface displacement amount, the second interface displacement amounts, and the reference displacement amount.
