An ultrasound diagnostic apparatus and a method of determining elasticity index reliability control transmission and reception of an ultrasonic beam, track displacements in a transmission direction of a plurality of points of a blood vessel in the transmission direction of the ultrasonic beam, calculate an elasticity index of the blood vessel based on the tracking result, acquire the direction of a displacement or the amount of displacement in the transmission direction of a specific point of the blood vessel, and determine reliability of the elasticity index based on the acquisition result. A program for determining elasticity index reliability causes a computer to execute the respective steps of the method. A computer readable storage medium stores the program.