PURPOSE: To provide a satisfactory B-mode image even though an ununiform sonic speed layer is present at a position distant away from an ultrasonic probe.

CONSTITUTION: The phase difference between elements are subjected to FFT so as to create a plurality of phase difference maps for periodic components (P74), and amplification degrees for the periodic components are obtained (P75) in view of the depth of an ununiform sonic speed layer. Then, the periodic components are amplified (P76), and thereafter, the phase difference maps are subjected to IFFT so as to obtain values of compensation for delay times of the elements (P77). Accordingly, even though an ununiform sonic speed layer is present in the living body, it is possible to provide a satisfactory B-mode image.