The present invention aims at providing an ultrasound image diagnostic apparatus capable of shortening the time required for measurement and calculation of an optimal sound velocity and obtaining an ultrasound image having excellent image quality at a tissue or lesion the operator desires to observe. There are provided an element data memory storing element data a region-of-interest setter setting a region of interest an optimal sound velocity calculator calculating an optimal sound velocity at the region of interest using the element data corresponding to the region of interest a reconstruction region setter setting a reconstruction region that contains the region of interest and is larger than the region of interest and an image reconstructor generating a reconstructed image by reconstructing the ultrasound image in the reconstruction region based on the optimal sound velocity.
