An ultrasound signal processing device: performing events involving transmitting ultrasound towards a subject; receiving ultrasound reflection from the subject in response to each event; and generating a frame signal from sub-frame signals generated based on the ultrasound reflection. The device, in each event, causes a transmission aperture to transmit ultrasound focusing in the subject. A transmission aperture for one event differs in position, in a transducer element array direction, from a transmission aperture for a previous event by a shift amount of at least one transducer element width. The device, for each event, sets a target area (Bx) which includes a position (F) where transmitted ultrasound focuses and whose width (W f ) in the transducer element array direction, at a depth where the transmitted ultrasound focuses, is equal to or greater than the shift amount. The device generates a sub-frame signal covering measurement points (P) included in the target area (Bx).