Without using any approximated wave front propagation model, appropriate delay times are set for received signals to obtain phased signals for both the inside and outside of irradiation area of transmission beam. A reception beamformer comprises a wave front propagation calculator 121 that obtains times until ultrasonic waves transmitted from a plurality of ultrasonic transducers arrive at a reception focus by calculation, and a delay time extractor 122 that calculates delay times for the reception focus on the basis of distribution of the arrival times of the ultrasonic waves for each of the plurality of the ultrasonic transducers obtained by the wave front propagation calculator 121. Therefore, even if the wave front that arrives at the reception focus has a complicated shape, it is not necessary to approximate the wave front, and phasing addition can be performed with appropriate delay times obtained from times until the ultrasonic waves arrive at the reception focus.
