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 a tomogram image 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; wherein when a part or all of five screens of an A cross-section, a C cross-section, an S cross-section, a cross-section of the developed image, and a cross-section of the rotated image are displayed, a user specifies a position on any one of the screens, the navigating means switches remaining screens to cross-sections which show the position specified by the user and show the positions with intersections of cross lines or a contacting part of a circle and a half lin.