To ensue the security function while keeping high versatility for an information storage medium by ciphering selectively the data received from a host device and decoding the stored data.

When a host device 2 writes the new file data into an information storage medium 1a, a central controller 3a identifies whether the written data are the file management data or the file data based on the logical address value that is designated by the device 2. If the file management data are identified, a ciphering/decoding circuit 9 is not started and accordingly the data are not processed and stored in a storage 4 as they are. If the file data are identified, the circuit 9 is started to perform a ciphering operation based on the key information set by a ciphering/decoding key setting circuit 10. Then the ciphered data are written into a prescribed address of the storage 4 via a storage control circuit 12. This operation is reversed when the file data are read out of the medium 1a.