An ultrasound diagnostic apparatus includes a light irradiator for irradiating a lattice point set in the region to be imaged with light converging to the lattice point, so as to impart heat locally to the point an image for distortion amount calculation-generating section for generating an ultrasound image for distortion amount calculation based on a reception signal of ultrasound generated and a distortion amount calculating section for calculating the difference between the position of the lattice point on an ultrasound image for distortion amount calculation and the absolute coordinates of the lattice point as the distortion amount. Such apparatus allows precise sound speed values in the living body to be obtained and, accordingly, an ultrasound image of high accuracy to be taken. In consequence, a more accurate diagnosis is conducted on the region to be diagnosed in a subject.
