PURPOSE: To increase the reliability of data, and to decrease the burden of a user by reading data from two memory areas by means of the read command of a host system, comparing the data, and detecting the generation of memory change.

CONSTITUTION: When an ID controller 1 receives the write command of a host system 2, a CPU 11 writes the data with same contents to the first and second memory areas of a data carrier 4. When the read command of the host system 2 is received, the data are read from the first and second memories, the both data are compared, when the both data are identified practically as the same, the data are decided to be suitable, and transmitted. When the identification of the data cannot be obtained, the memory change, which means one of the data is erroneous, is decided, and it is responded to. Thus the reliability of the data is increased, and the burden of the user is decreased.