1. A periodontal disease testing apparatus comprising: an X-ray CT scanner photographing a portion including a tooth, a gingiva, and alveolar bone within an oral cavity to generate volume data of the portion; region setting means setting a related region of a subject tooth based on the volume data; defining means defining a principal axis of the subject tooth; correcting means correcting the related region so as to align the principal axis of the subject tooth with a normal direction of a cross-section of an X-ray CT image; multi-planar reconstruction image generating means generating a multi-planar reconstruction image of the corrected related region; means for generating developed image and the like transforming a coordinate of the related region to generate a developed image and a rotated image; and navigating means indicating correspondences of a spatial coordinate among the multi-planar reconstruction image, the developed image, and the rotated image.