An ultrasound diagnostic apparatus includes: an ultrasound probe including a plurality of ultrasound transducers; a device for determining optimum sound speed values at a lattice point set in a shallower region than a region of interest (ROI) and in the ROI on the basis of an ultrasound detection signal outputted by the ultrasound probe; a device for arithmetically operating a received wave received from the ROI on the basis of the optimum sound speed value; a device for setting an assumed sound speed in the region of interest, obtaining a received wave received from each lattice point on the basis of the assumed sound speed and the optimum sound speed value at the lattice point, and synthesizing received waves at lattice points to obtain a resultant received wave; and a device for determining a local sound speed value in the ROI on the basis of the received wave and the resultant received wave.