Abstract: A magnetic card discriminator which is equipped with a card reading part, a registration number memory part, an invalid number memory part, a processing part, and an alarm, and which discriminates whether a card registration number magnetically recorded on a magnetic card is an invalid registration number or not; wherein the card reading part reads the card registration number, the registration number memory part stores the foregoing card registration number having been read, the invalid number memory part has previously stored an invalid registration number of a magnetic card not to be used, the processing part compares the coincidence and non-coincidence of the card registration number having been read out from the registration number memory part with the invalid registration number, and actuates the alarm on the basis of the result of the above comparison such as, for example, the coincidence, and then the alarm makes an alarm signal in the combination of, for example, sound and light.