Abstract: A two-dimensional echogram or a tomogram (B-mode image) and a volume of an organ are displayed in real time while radiating an ultrasonic wave to the organ. The volume of the organ is obtained by first designating a plurality of line segments traversing the image of the organ on the two-dimensional echogram, by obtaining a pair of intersections between each line segment and the wall of the organ by using a signal intensity profile (A-mode image) of each line segment, by obtaining a length between the pair of intersections, and by calculating the volume by the Simpson method by using the obtained length. In order to improve the precision of volume calculation, the signal intensity profile is smoothed and the intensity is normalized. The intersections are displayed superposed upon the two-dimensional echogram to allow an operator to visually check easily whether the intersections are correctly aligned with the wall of the organ.