A diagnostic ultrasound apparatus includes a displacement image data frame memory for storing, frame by frame, two-dimensional ultrasound image data of a living body to be observed; displacement image data sampling section for sampling displacement image data which is obtained by comparing two-dimensional ultrasound image data of a newest frame with the two-dimensional ultrasound image data of any one of the previous frames stored in the displacement image data frame memory, the displacement image data representing displacement in the living body between the compared frames; displacement hysteresis image forming section for forming displacement hysteresis image data which represents changes between the thus-formed plural displacement image data by sequentially combining the plural displacement image data with the lapse of time; and a display monitor for displaying a displacement hysteresis image on the basis of thus-produced displacement hysteresis image data. When this apparatus is used for diagnosing a heart, the displacement hysteresis image shows the state of movement of the living body such as the heart muscle with the lapse of time for the time interval from peak expansion of the heart ventricle to peak contraction thereof. In this case, since the width of the displayed displacement hysteresis image shows the amount of movement (activity) of the heart muscle, there is improved accuracy in diagnosing such things as abnormal movement in the heat and the position where the abnormal movement occurs.