An ultrasound signal processing device performs transmission events by an ultrasound probe having transducer elements, generates a sub-frame acoustic line signal for each transmission event based on reflected ultrasound, and synthesizes sub-frame acoustic line signals to generate a frame acoustic line signal. A transmitter causes the ultrasound probe to transmit ultrasound beams to an ultrasound main irradiation area. A receiver generates receive signals for the transducer elements. A delay-and-sum calculator sets a target area in the ultrasound main irradiation area, and performs delay-and-sum calculation with respect to the receive signals from measurement points in the target area to generate a sub-frame acoustic line signal. The target area has, in a depth range shallower than a focal point, a width in the transducer element array direction that decreases as the depth decreases. A synthesizer synthesizes sub-frame acoustic line signals.