PURPOSE: To securely detect an illegal card by writing use state code in a use state confirmation area each time the card is used, confirming the use state code, and comparing it with the contents of a balance code area and judging whether or not the card is illegally used.

CONSTITUTION: Data 2 for state confirmation are read out and the matching judgement process of the card is performed. This process is performed by confirming data 2 for state confirmation stored in the use state confirmation area 3 with the contents of a balance code area written on the card 1 by reading write information by a read head. When the frequency of use indicates 0, the state confirmation data 2 is 0 and nothing is written in the corresponding balance code area. When it is judged from the collation result that the state confirmation code 2 and the information in the balance code area match each other, the card is judged to be normal and the balance is rewritten. When not, on the other hand, the card is judged to be an illegal and the read process is quit.