A speed evaluation unit in the present invention specifies measurement values that satisfy rejection conditions from among a plurality of measurement values. A propagation velocity Vs for which the sign is minus (reverse direction) is set as a subject to be rejected under condition 1. The propagation velocity Vs which is outside a threshold value range is set as a subject to be rejected under condition 2. The propagation velocity Vs at each depth in which fluctuation was detected is set as a subject to be rejected under condition 3. Specifying the changes in phase shift that accompany the passage of shear waves in regions (depths) at which fluctuations occur is difficult, so measuring the propagation velocity Vs of shear waves is difficult. Supposing that the propagation velocity Vs of shear waves in the regions (depths) at which fluctuations occur could be measured, the reliability of the measurements would be a concern. This is why the propagation velocity Vs at each depth in which fluctuations were detected is set as a subject to be rejected as a result with low reliability.
