Forgery card discrimination system, device for writing genuine-fake judgment information and forgery card discrimination device

A counterfeit card detecting system preferable to prevent false use of a counterfeit card at low cost while enabling use of magnetic recording cards such as existing cash cards and credit cards just as they are, an authentication judgment information writing device, and a counterfeit card detecting device are provided. The counterfeit card detecting system (1) comprises a magnetic card reading unit (1a), a magnetic card judging unit (1b), and a true signal (analog signal) write unit (1c). All track data on a magnetic card is read as an analog signal and the waveform of the analog signal is analyzed. The magnetic card is judged if it is anauthentic one or a counterfeit one. If the object magnetic card is an authentic one, analog signal data according to the unique algorithm is written in the portion before the STX signal write location and in the portion after the ETX signal write location.