A system for comparing dental X-ray images includes a positional displacement calculator calculating a positional displacement between dental X-ray test and reference images by using phase-only correlation, a positional displacement corrector correcting the positional displacement, a base point extractor defining, as a base image, any one of the dental X-ray test and reference images, and defining, as a corresponding image, the other one of the two dental images, and extracting base points from the base image, a corresponding point extractor extracting corresponding points, which correspond to the base points, from the corresponding image, a correspondence calculator calculating correspondence between the base points and the corresponding points, a nonlinear distortion corrector correcting a nonlinear distortion between the base image and the corresponding image, based on the correspondence, and a similarity calculator finding, by using phase-only correlation, a similarity between the base image and the corresponding image.
