A plurality of opening positions is determined along a θ direction which is a curved direction on a two-dimensional transducer element array. Transmission openings are sequentially set to the plurality of opening positions. Transmission beam deflection scanning is performed at each opening position using the transmission openings. Transmission beam lines radiating from the center of the transmission opening are thus formed. A transmission apodization is applied before and after switching from one opening position to another so that there are no steps in the transmission sound field.