A beamforming method according to the present invention is a method of processing echo signals of a target region which are obtained from a probe including a plurality of receiving elements arrayed on a predetermined line. The beamforming method includes the following steps (S1 to S3). At S1, seed beams are formed from echo signals received by at least two receiving elements from among the plurality of receiving elements. At S2, a main beam and sub beams are formed by synthesizing at least one of the seed beams. At S3, a narrow beam for the target region is formed by multiplying the sub beams by respective predetermined coefficients and subtracting the multiplied sub beams from the main beam. Here, an signal intensity for the target region regarding the main beam is higher than a signal intensity for the target region regarding each of the sub beams.
