PURPOSE: To prevent the illicit use of the codes of a reader/writer which can read and write the data after evaluating the justice of the data carrier to be used with comparison of the codes.

CONSTITUTION: A 1st master code is registered in a reader/writer 10, and a 1st code is registered in a key carrier 20 respectively. A 2nd code is also registered in the carrier 20. In the access modes of the reader/writer 10 and the carrier 20, the 1st master code of the reader/writer 10 is compared with the 1st code of the carrier 20. When the evaluating result showing the just use of the carrier 20 is obtained, the 2nd code is read out of the carrier 20 and registered in a memory 16 of the reader/writer 10 as a 2nd master code. At the same time, the 2nd master code of the reader/writer 10 is compared with the 2nd code of the carrier 20 also in the access modes of the reader/writer 10 and the carrier 20. Then an access is allowed to the carrier 20 when the just evaluating result of the carrier 20 is obtained.