PURPOSE: To leave an echo in an object, and to remove reverberation by obtaining a C-mode output picture-element value by the product of the C-mode image of each picture element and a C-mode threshold acquired from a vertical threshold and a horizontal threshold.

CONSTITUTION: A reception signal from an object O in an echo sounder transmitter-receiver 1 is processed in a signal processing circuit 3 through a transmitter-receiver 2. Received C-mode input picture-element values G (i, j) are stored in a memory first. The mean value AVV of picture elements in the row direction of each vertical direction are obtained, and compared with the values of picture elements in each row direction, and vertical thresholds V (i, j) are arithmetically operated in response to the results of the comparison. Likewise, horizontal thresholds H (i, j) are arithmetically operated. C-mode thresholds S (i, j) are arithmetically operated by the AND of the vertical thresholds and the horizontal thresholds. C-mode output picture-element values G' (i, j) are arithmetically operated by the products of the stored C-mode input picture-element values G (i, j) and the C-mode thresholds S (i, j).