In a fish-finder of the type emitting ultrasonic pulses into the water and receiving and displaying the reflected waves, the received reflected waves are converted into digital signals corresponding to their levels and written into a main memory as information of one display line of a color cathode ray tube display. The main memory has a capacity of storing information of one picture frame of the color cathode ray tube display, and is read out repeatedly. The read-out signals are converted into color signals according to their levels and the color signals are supplied to the color cathode ray tube. Every time a new reflected wave is written into the main memory, the stored content of the main memory is partly removed so that the oldest display on the screen may disappear and, at the same time, the display on each display line is shifted to the older display line.