A region-of-interest and grid point setting unit is configured to set a region of interest (ROI) within an object and grid points to be measured within the ROI in the depth direction. An intra-region-of-interest sound velocity calculation unit is configured to calculate ambient sound velocities at the grid points at respective depths of the ROI. A local sound velocity determination unit is configured to compare the values of an ambient sound-velocity distribution table for each plurality of assumed local sound velocities with the values of the ambient sound velocities calculated by the intra-region-of-interest sound velocity calculation unit, extract an ambient sound-velocity distribution table for assumed local sound velocities having the smallest error, and determine the assumed local sound velocities of the extracted ambient sound-velocity distribution table as local sound velocities in the ROI.
