A received signal is obtained along an ultrasonic beam while the ultrasonic beam which passes through the measurement area is moved in periodic fashion inside the measurement area. A sampling processing unit (20) uses a plurality of sampling sets mutually offset in time phase, and samples received signals across a plurality of time phases for each sampling set to thereby obtain a received data string for each sampling set. Doppler information in the measurement area is then obtained on the basis of the plurality of received data strings that correspond to the plurality of sampling sets.
