A beamforming method of processing echo signals of a target region, the echo signals being obtained from a probe including a plurality of receiving elements arrayed on a predetermined line, the beamforming method comprising: forming seed beams from echo signals received by at least two receiving elements from among the plurality of receiving elements, the forming of the seed beams including multiplying the echo signals received by the at least two receiving elements by a window function with a predetermined time width and a predetermined intensity, and forming the seed beams from the multiplied echo signals; forming a main beam by performing weighted-beam multiplication by using at least one of the seed beams, and forming sub beams by performing weighted-beam addition by using the main beam and at least one of the seed beams; and forming a narrow beam for the target region, by multiplying the sub beams by respective weighting coefficients determined by learning and subtracting the multiplied sub beams from the main beam, wherein the seed beams formed in the forming of the seed beams have different signal characteristics, by using at least two kinds of combinations of receiving elements from among the plurality of the receiving elements, in the forming of the main beam and the sub beams, the main beam and the sub beams are formed such that a 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, and in the forming of the narrow beam, the respective weighting coefficients of the sub beams are determined in accordance with change of a directivity of the narrow beam that is formed by changing the weighting coefficients. forming seed beams from echo signals received by at least two receiving elements from among the plurality of receiving elements, the forming of the seed beams including multiplying the echo signals received by the at least two receiving elements by a window function with a predetermined time width and a predetermined intensity, and forming the seed beams from the multiplied echo signals; forming a main beam by performing weighted-beam multiplication by using at least one of the seed beams, and forming sub beams by performing weighted-beam addition by using the main beam and at least one of the seed beams; and forming a narrow beam for the target region, by multiplying the sub beams by respective weighting coefficients determined by learning and subtracting the multiplied sub beams from the main beam, wherein the seed beams formed in the forming of the seed beams have different signal characteristics, by using at least two kinds of combinations of receiving elements from among the plurality of the receiving elements, in the forming of the main beam and the sub beams, the main beam and the sub beams are formed such that a 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, and in the forming of the narrow beam, the respective weighting coefficients of the sub beams are determined in accordance with change of a directivity of the narrow beam that is formed by changing the weighting coefficients.