1. A system for comparing dental X-ray images comprising: a positional displacement calculator configured to calculate a positional displacement between a dental X-ray test image and a dental X-ray reference image, by using phase-only correlation; a positional displacement corrector configured to correct the positional displacement; a base point extractor configured to define, as a base image, any one of the dental X-ray test image and the dental X-ray reference image between which the positional displacement is corrected, to define, as a corresponding image, the other one of the two dental images, and to extract a plurality of base points from the base image; a corresponding point extractor configured to extract a plurality of corresponding points, which correspond to the plurality of base points, from the corresponding image; a correspondence calculator configured to calculate correspondence between the plurality of base points and the plurality of corresponding points nonlinearly distorted from the plurality of base points; a nonlinear distortion corrector configured to correct a nonlinear distortion between the base image and the corresponding image, based on the correspondence; and a similarity calculator configured to find, by using phase-only correlation, a similarity between the base image and the corresponding image between which the nonlinear distortion is corrected.