A Doppler sensitivity measuring unit measures the Doppler sensitivity of Doppler spectrum data obtained by ultrasonic wave transmission/reception with respect to a predetermined region of an object to be examined. A spectrum shape model generating unit generates a spectrum shape model on the basis of a window function used when the above Doppler spectrum data is generated. A threshold setting unit sets a threshold range in which trace waveform data displaces by a predetermined amount in the frequency axis direction on the basis of the spectrum shape model and the Doppler sensitivity, and sets a predetermined number of thresholds at almost equal intervals in the threshold range. A trace data generating unit selects a threshold for the acquisition of desired trace waveform data by generating trace waveform data while sequentially updating the plurality of set thresholds.