Reception beamforming is executed using delay times that complexly vary in accordance with differences between transmission conditions. An irradiation area 32 of a transmission beam is calculated, and the lengths of segments, using which delay times are calculated, are set in accordance with the positional relationships between the calculated irradiation area 32 and reception scanning lines 31. For example, the reception scanning lines 31 are divided into areas A to C, and the lengths of segments 40b in the outer area B located outside of the irradiation area 32 are set shorter than the lengths of segments 40a and 40c in the inner areas A and C.