A beamforming method includes: generating a main beam signal by performing, with a focus on a first region, a delay and sum operation on receiving echo signals; generating a sub beam signal which has a low sensitivity to ultrasound signals reflected off the first region; and generating a narrow beam signal by (i) calculating a coefficient for narrowing an angle of the main beam signal, and (ii) multiplying the main beam signal by the coefficient, wherein in the generating of a sub beam signal, the sub beam signal is generated using a differential signal that is a difference between two beam signals each of which is generated by performing a delay and sum operation on the receiving echo signals, with a focus on a corresponding one of two regions of a subject which are different from the first region and are different from each other.