A scanner transmits and receives ultrasonic waves at a specified pulse repetition frequency (PRF). A storage stores received signals acquired through the transmission and reception. A calculator generates a Doppler spectrum image by executing frequency analysis on the received signals. A display displays the Doppler spectrum image. When a desired Doppler velocity range for the displayed Doppler spectrum image is inputted, a processor reads out the received signals from the storage, and executes a resampling process on the read-out received signals at a sampling frequency corresponding to the desired Doppler velocity range. The calculator generates a new Doppler spectrum image by executing frequency analysis corresponding to the desired Doppler velocity range on the received signals having been subjected to the resampling process by the processor. The display displays the new Doppler spectrum image.
