An ultrasound signal processing device includes a transmitter that causes transducers to transmit an ultrasound beam, a receiver that, based on reflected waves received by reception transducers, generates reception signal sequences corresponding to the reception transducers, and a delay-and-sum section that, for a reference observation point in a region of interest, calculates delay times of reflected wave arrival to each of the reception transducers from the reference observation point as reference delay times and generates acoustic line signals by using the reference delay times corresponding to the reception transducers. For one or more dependent observation points in the region of interest that are contiguous in a depth direction from the reference observation point, the delay-and-sum section generates acoustic line signals by applying the reference delay times corresponding to the reception transducers.