An ultrasound diagnostic device to which an ultrasound probe having a plurality of transducer elements is connectable. The ultrasound diagnostic device includes ultrasound signal processing circuitry operating as: a delay-and-sum processor; a determiner; and a velocity value calculator. The determiner determines whether adjustment of a velocity value for a partial area in a subject is necessary, based on acoustic line signal intensities for a specific measurement point in the partial area and other measurement points in the partial area. The specific measurement point is selected from among measurement points in the partial area based on acoustic line signals for the measurement points in the partial area. The velocity value calculator, when the determiner determines that velocity value adjustment is necessary, calculates an adjusted velocity value for the partial area by using the acoustic line signal for the specific measurement point.