A measurement data processing device measures a size of a diagnostic region from a tomographic image as two-dimensional information. An ROI width determining device determines a size of a region of interest based on the size (lateral width and longitudinal width) of the diagnostic region. An oscillation angle determining device determines a range of an angle for oscillating ultrasonic transducers based on the size. An oscillation rate determining device determines a rate for oscillating the ultrasonic transducers based on information indicating the oscillation angle and image quality. In order to acquire a three-dimensional image of the diagnostic region, the probe oscillation control device controls the oscillation of the ultrasonic transducers based on the information. A three-dimensional image processing device extracts a three-dimensional image of in the determined region of interest from the acquired three-dimensional image. A display device displays the extracted three-dimensional image. Then, the three-dimensional image based on the shape of the diagnostic region is acquired.