PURPOSE: To provide a beam forming device with a simple construction in an ultrasonic device without deteriorating the quality of the image. CONSTITUTION: A beam forming device of a PASS ultrasonic imaging device includes a set of σ.δ modulator 201, which operates to digitize separately the echo signals received from each converter element. 1-bit digital displays of each echo signal excessively sampled are delayed, if necessary, for determination of direction and focusing and are aggregated. A subtrahend filter 213 slows down the speed of digitized received beam samples before the same displays images of the received beam.