A processor generates a first Doppler spectrum image representing the velocity of a moving body at each observation point by analyzing the frequency of signals received at the plurality of observation points acquired via Doppler scanning. An interpolator acquires a second Doppler spectrum image consecutively at each observation point time when the ultrasound has not been transmitted to and received from each observation point by the Doppler scanner transmitting and receiving the ultrasound toward and from the plurality of observation points at a plurality of respective times, based on interpolation of the first Doppler spectrum image at each observation point. The interpolator generates a third Doppler spectrum at each observation point by combining the first Doppler spectrum at each observation point and the second Doppler spectrum image at each observation point per observation point.