1. An image processing method comprising steps of: extracting, using a processor, a medullary cavity region comprising a hollow region inside a joint from a digitalized image of a cross section of examined joint bones only, using an Expansion and Shrinkage method, and further using the processor; filling the medullary cavity region by figuring out a sum of the extracted medullary cavity region and the digitalized image of the cross-section of the examined joint bones; extracting a surface structure of the joint bones, whereby generating a digitalized image of the cross-section of the examined joint bones; stacking a plurality of the digitalized images of a plurality of the digitalized images of a plurality of the cross sections of the examined joint bones so as to obtain a 3D image; performing 3D labeling for the 3D image so as to define a 3D joint image to be evaluated; and judging, identifying, and extracting a joint image to be evaluated from correspondence of centers of gravity positions of each of the joint bones.