PURPOSE: To attain to reduce the side lobe level of a re-constituted image and to enhance azimuth resolving power over a wide range in a distance direction, in performing imaging processing, by compressing the amplitude of a hologram, and expanding the amplitude of an image signal.

CONSTITUTION: Ultrasonic beam is emitted into a living body in a converged state from a probe 1 energized by a pulser 2 and the reflected wave from the living body is received in wide sensitivity distribution. Hologram data of orthogonal detection output are converted by A/D converters 6, 7 while digitized data are stored in image memory 11 through a computer CPU 10 and subsequently transmitted to buffer 12. In this case, CPU 10 performs the compression of the amplitude of the hologram data and read canal data corresponding to the compression from ROM to perform the integration and adding calculation of each data while the absolute value of the calculated result is calculated and the amplitude thereof is expanded according to a square function and the obtained data is stored in the address of memory 13 corresponding to a transmission position and a depth. By this method, all re-constituted images are calculated and imaging information is obtained.