1. A three dimensional abdominal cavity region extracting apparatus, comprising: bone region extracting means, for extracting bone regions that represent the bones of a subject within a plurality of axial tomographic images including at least a first axial tomographic image that includes the ribs of the subject, a second axial tomographic image that includes the vicinity of the upper edge of the pelvis of the subject, and a third axial tomographic image that includes the vicinity of the pelvis of the subject lower than that which appears in the second axial tomographic image, obtained from a three dimensional image that represents a portion of the subject from the vicinity of the upper end of the liver to the vicinity of the pubic symphysis; bone boundary point detecting means, for detecting a plurality of bone boundary points that represent 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 abdominal cavity region extracting means, 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, based on the bone boundary points, and for extracting a three dimensional abdominal region surrounded by the curved surfaces as a three dimensional abdominal cavity region.