PURPOSE: To generate a hysteretic echo signal which corresponds to the sweep speed of a cathode-ray tube, by generating a random-number signal having hysteresis at a random-number generation part, then eliminating the use of a memory.

CONSTITUTION: A random-number generation part 1 using a microprocessor, etc., while generating a random-number signal used as an irregular echo signal, generates another random-number signal used as a hysteretic echo signal. The same data held in a register or small-capacity memory in an echo image generation part 2 is supplied to the random-number generation part 1 simultaneously with a control signal. The generation part 2 uses the random-number signal from the random-number generation part 1 as the hysteretic echo signal and synthesized with the irregular echo signal, then the resulting signal is outputted to a display control part 4. On a cathode-ray tube 5, the signal based upon both the echo signals is displayed.