The analysis image generating unit generates section images from volume analysis data that is collected by sending an ultrasound wave down to a region under the ribs. The right/left identifying unit identifies the right or left breast from cyclic motion components in the section images. The extending direction detecting unit analyzes plane-A images or plane-B images generated from the same volume analysis data, or a plane-C thickness-added MIP image, and detects the rib extending direction. The extending direction detecting unit also determines the position of the ultrasound probe based on the relative displacement of the extending direction. The body mark generating unit generates a body mark from the analysis results obtained by the right/left identifying unit and the extending direction detecting unit. The image synthesizing unit integrates the display image generated by the display image generating unit and the body mark, and displays it on the monitor.