The beam forming method comprises: a main beamforming step (S21) which generates a main beam signal by phased addition of the received echo signals obtained from multiple reception elements with a first region of the subject as a focal point a secondary beamforming step (S22) that generates from the received echo signals a secondary beam signal which has a lower sensitivity than the main beam signal to ultrasonic wave signals reflected from the first region and a narrow beamforming step (S23) which calculates a coefficient for narrowing the angle of the main beam signal the coefficient being determined on the basis of the main beam signal and the secondary beam signal and generates the narrow beam signal by multiplying the coefficient and the main beam signal. In the secondary beamforming step (S22) the secondary beam signal is generated using a difference signal which is the difference between two beam signals that are generated by phased addition of the received echo signals with each of two regions of the subject which differ from the first region and from each other as the focal points.
