1. An ultrasonic image boundary extracting method comprising the steps of: (a) obtaining an interrelationship among plural reception signals with respect to a region within an object to be inspected from among reception signals obtained by transmitting ultrasonic waves toward the object from plural ultrasonic transducers and receiving ultrasonic waves reflected from the object; and (b) detecting a boundary between plural different tissues within the object based on said interrelationship to thereby generate information on a tissue boundary.