In this ultrasound diagnostic device and ultrasound diagnostic image data processing method, an element data determination unit compares a pre-set threshold value with the average value of the amplitude values in the element direction of the element data calculated by an average value calculation unit in the element direction, and an RF data calculation unit performs a process in which the element data of which the absolute value of the amplitude value is the maximum is adopted as the RF data without phasing addition when the average value (Aa) is greater than the threshold value. Meanwhile, when the average value (Ab) is equal to or lower than the threshold value, the RF data calculation unit performs a process in which the average values of all the amplitude values in the element direction of the element data are adopted as the RF data without phasing addition.
