To provide a method for deciding an aperture of ultrasonic diagnostic equipment wherein a picture of an excellent quality can be obtained by preventing the deterioration of the shape of a beam.

The method comprises: a beam forming simulation process S102 which includes the adding method and the thinning method of a received signal from a vibrator and performs the simulation of the shape of a formed ultrasonic beam based on a parameter about the signal processing condition of a probe; and an added and thinned shape judging process S103 for judging whether the shape of the beam obtained by the beam forming simulation process meets a predetermined judging reference including the maximum value of a side lobe level, the tolerance of sensitivity lowering with respect to a case of including no adding and thinning, etc.