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 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 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, characterized in that the bone boundary point detecting means sets search reference lines within each of the plurality of axial tomographic images, the reference lines having one end point of each side formed by bisecting the subject in each axial tomographic image in the vertical direction, the end points being at positions which are in the abdominal cavity region, such that the distances between the end points become shorter in axial tomographic images that represent lower portions of the subject, searches along a plurality of lines of sight that do not intersect and extend from a plurality of points along the search reference lines toward the exterior of the subject, and detects points on the lines of sight that contact the bone regions first as the bone boundary points. bone region extracting means, for extracting bone regions that represent the bones of a subject within a plurality of axial tomographic images 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 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, characterized in that the bone boundary point detecting means sets search reference lines within each of the plurality of axial tomographic images, the reference lines having one end point of each side formed by bisecting the subject in each axial tomographic image in the vertical direction, the end points being at positions which are in the abdominal cavity region, such that the distances between the end points become shorter in axial tomographic images that represent lower portions of the subject, searches along a plurality of lines of sight that do not intersect and extend from a plurality of points along the search reference lines toward the exterior of the subject, and detects points on the lines of sight that contact the bone regions first as the bone boundary points.