When a subject is pressed by a plate and an acoustic wave is received by a probe via the plate, the acoustic wave is refracted because of the difference between a sound velocity in the subject and a sound velocity in the plate. When the refraction is not considered, a reduction in resolution occurs. Weights for signals or virtual wavefronts corresponding to the signals to be added to one another are determined using a thickness of the plate, the sound velocity in the plate, the sound velocity in the subject, positions of image elements (pixels or voxels) for image information, and arrival times of the acoustic wave from an object that is a sound source. The signals or the virtual wavefronts corresponding to the signals are added to one another, thereby acquiring the image information.