To prevent magnetic recorded data on a magnetic prepaid cared from being erased and rewritten by applying a magnetic coating onto a made material, encoding the magnetic recorded data in the not dried state of a magnetic coating film, immediately drying that film, and fixing the magnetic recorded data.

The magnetic ROM card is composed of a card base material 1, magnetic recording layer 2 and fixed end mark 3. First of all, the magnetic coating is applied onto the card base material 1, the magnetic recorded data are encoded in the state where the magnetic coating film is not dried, then that film is immediately dried, and the magnetic recorded data are fixed. Thus, the magnetic recorded data are prevented from being erased and rewritten, at the same time, the fixed magnetic recorded data have frequency signals or the start mark and end mark of the fixed magnetic recorded data, and the magnetic recorded data are provided between both the marks. Thus, the illegal use of a used magnetic prepaid card is prevented.