The outside region of interest and inside region of interest are set automatically in the soft tissue area on the basis of the coordinate information of the reference region of interest when a reference region of interest is set manually on a bone density image. The average fat percentage of each region of interest is calculated from the fat percentage distribution within these regions of interest. A second outside region of interest may be set on the outside of the greater trochanter, and the average soft tissue thickness within that range may be calculated.