PURPOSE: To display an ultrasonic image even after a display image is frozen by equipping a digital scan converter with at least four image memories and a coefficient changeover memory to execute the processing such as a correlation and an interpolation between image data read from the memories and to output the result to a display device.

CONSTITUTION: A DSC 3 is equipped with image memories 5a-5d to individually store secular change image data, simultaneously, a coefficient changeover memory 6 to execute the processing such as the correlation and interpolation of the data between the image data read from the memories 5a-5d and to output the result to a display device 4, and simultaneously, the DSC 3 is equipped with a control part 7 to independently control the write and read of the data to the memories 5a-5d. The control part 7 successively writes the newest data outputted from an ultrasonic transmitting and receiving part 2 and the data to be old one before them to the individual memories 5a-5d, simultaneously, respective data are read from the memories 5a-5d, the processing of the correlation, interpolation, etc., is executed from both data, image data are prepared, and they are displayed on the display device 4.