An ultrasound signal processing device: performing transmission events each involving selecting a group among transducer elements arranged in line in an ultrasound probe, and causing the group to transmit ultrasound towards a subject for each transmission event, receiving ultrasound reflection from the subject and generating a frame acoustic line signal based on sub-frame acoustic line signals that are generated based on the ultrasound reflection. The ultrasound signal processing device includes: a calculator that calculates, based on reflected ultrasound, a motion amount indicating movement between image signals for the subject and a synthesizer generating the frame acoustic line signal based on the sub-frame acoustic line signals. The number of sub-frame acoustic line signals that the synthesizer uses to generate a frame acoustic line signal decreases when the motion amount is equal to or greater than a predetermined threshold.
