To provide an authenticity judging device which improves an accuracy of the judging authenticity of a medium and sufficiently prevents a forgery of the medium by judging the authenticity of the medium as to whether a mag netic material which is mixed, coated or sandwiched to the medium is appropri ate or not.

The authenticity judging device 10 obtains a hysterisis characteristics of the magnetic material 2 mixed, coated, or sandwiched to a medium 1, by periodically changing an intensity of a bias magnetic field generated by a magnetic generating part 13. By using the hysterisis characteristics, it is possible to judge whether the magnetic material 2 mixed, coated or sandwiched to the medium 1 is appropriate or not, and to improve the accuracy of the judging authenticity of the medium 1. Also, since the magnetic material 2 of the same material as the magnetic material 2 mixed, coated, or sandwiched to an authentic medium is required in order to forge the medium 1, the forgery of the medium 1 can be rendered difficult so as to sufficiently prevent the medium 1 from forging.