To reduce the circuit scale while maintaining the quality of images in a digital beam forming chip where an AD converter used to form a digital beam for receiving of an ultrasonic diagnostic apparatus and a digital phasing adder are both present.

In the ultrasonic diagnostic apparatus which receives ultrasonic waves reflected inside a subject by a plurality of ultrasonic vibrators, a signal from each ultrasonic vibrator is converted to a digital signal by the AD converter and added after phased by a digital phaser to phase the received ultrasonic signal. As the intensity of the received ultrasonic waves significantly varies according to subjects, the quality of an image is good if the number of bits converted by the AD converter is large but the circuit scale of the digital phaser and the adder is large. Accordingly, a bit selector with a function of using upper bits in the output of the AD converter when the input signal is large while using lower bits when the input signal is weak is inserted to control the number of bits processed by the digital phaser and the adder, so that the circuit scale is reduced.