[Objective]To detect an abdominal cavity region of a subject in a three dimensional manner with higher accuracy.[Constitution]An apparatus is provided with: a bone region extracting section (13) for extracting bone regions representing bones of a subject within a plurality of axial tomographic images obtained from a three dimensional image representing a portion of the subject from the vicinity of the upper end of the liver to the vicinity of the pubic symphysis a bone boundary point detecting section (16) for detecting a plurality of bone boundary points representing the boundaries between the detected bone regions and regions positioned toward the interiors of the bone regions within the plurality of axial tomographic images and an abdominal cavity region extracting section (18) for estimating curved surfaces within the three dimensional image that substantially contact the interiors of the plurality of bone boundary points detected in each of the plurality of axial tomographic images, and for extracting a region surrounded by the curved surfaces as a three dimensional abdominal cavity region.
