PURPOSE: To enable the comparative examination of left-hand and right-hand information without dispersing sight line, by separating a screen to the left and to the right to display two pieces of information side by side near the center thereof.

CONSTITUTION: A boat is moved over a school of fish. When the school of fish is large as shown by Figs (a) and (b) while the moving speed thereof is small, in order to determine the moving direction thereof and left and right distribution states thereof from an image as shown by Fig. (c), a display screen is divided into the left and right portions as shown by Fig. (d) and images in patterns indicating a reflected wave are fed on a display screen, to the left at the left portion as shown by the arrow P and to the right at the right portion as shown by the arrow S. In this manner, as for the latest information, left-hand information is displayed on Po and right-hand information on So. The latest information is shown at the center position of the image and the oldest information at both ends thereof. Thus, the images are fed at the left portion of the screen to the left from the center thereof and at the right portion thereof to the right therefrom thereby enabling comparative examination of left-hand and right-hand informations without shifting sight line to an image near the center.