Based on generated tomographic images, a first feature quantity representative of a clearness of sectional shapes crossing the Z′ axis at the vertebrae is calculated for each of respective points on the Z′ axis. Further, based on the generated tomographic images, a second feature quantity representative of a clearness of sectional shapes parallel to the Z′ axis at the vertebrae is calculated for each of the respective points on the Z′ axis. A third feature quantity representative of a regularity of an array of the vertebrae is calculated for each of the respective points on the Z′ axis, based on the calculated first feature quantity and the calculated second feature quantity. Positions of the vertebrae on the Z′ axis are calculated based on the third feature quantity, which is calculated for each of respective points on the Z′ axis.