Preceding decoders each convolve a corresponding one of a plurality of transducer elements constituting a transducer element array of an ultrasound probe with an impulse response waveform, while varying a filter coefficient corresponding to time side lobes for each transmission event. A succeeding decoder stores, in a memory, a reception beam signal that is output from a reception beam former. When a new reception beam signal is output, the succeeding decoder performs delay-and-sum on the new reception beam signal and the reception beam signal, which has been output immediately before the new reception beam signal and is stored in the memory.
