There is provided an automatic adjustment function of the dynamic range that carries out statistic processing by the use of each brightness of image data that corresponds to a plurality of frames, automatically determines the upper limit of the dynamic range based ON the brightness that corresponds to an echo signal from a subject, and automatically determines a lower limit value of the dynamic range based ON the brightness that corresponds to a noise level. By this function, in the case where the dynamic range first established by MANUAL operation and etc. is excessively wide or excessively narrow and the dynamic range can be automatically adjusted to an optimal setting even when the brightness distribution ON the image is dynamically varied.
