Provided is an apparatus of determining the number of synthetic beams, comprising: a motion measurement unit which measures the degree of motion in an output beam-forming image; a synthetic beam number determination unit which determines the optimum number of synthetic beams based on the measured degree of motion in case of applying the synthetic aperture beam forming; and a display unit which displays the number of synthetic beams determined by the synthetic beam number determination unit. A system using synthetic aperture beam forming detects motion in an image and displays a degree of the motion in various manners, and thus, a user is allowed to actively cope with the motion by adjusting the number of synthetic beams, or the system is allowed to immediately change the number of synthetic beams with reference to data stored in advance.