An ultrasound signal processing device includes ultrasound signal processing circuitry that operates as: a transmitter that causes an ultrasound probe to transmit ultrasound beams to an ultrasound main irradiation area defined by two straight lines each connecting a focal point and a different end of a transmission transducer element array； a receiver that generates receive signal sequences； a delay-and-sum calculator that sets first and second target areas in the ultrasound main irradiation area, and performs delay-and-summing of receive signal sequences based on ultrasound reflection from measurement points in the first and second target areas thereby to generate sub-frame acoustic line signals, the first target area is an entirety of an area located at or shallower than a focal depth, the second target area is part of an area located deeper than the focal depth； and a synthesizer that synthesizes sub-frame acoustic line signals to generate a frame acoustic line signal.
