Abstract: A method of protecting data in an IC memory card (11) having a ROM area (11a) and a RAM area (11b) and used as an external storage medium of a computer system includes storing key data for preventing copying in the ROM area and using the key data to prevent the copying of data stored in the RAM area. For example, when a file is written, second key data (k1, k2, . . . ), which is inserted into the headers (H1, H2, . . . ) of files (11c-1, 11c-2 . . . ), is decided in such a manner that a fixed correlation will exist with respect to first key data (K1, K2, . . . ) stored in the ROM area (11a), and the second key data is written in the RAM area (11b). When a file is read out of the IC memory card, a check is performed to determine whether the fixed correlation holds between the first key data that has been written in the ROM area and the second key data that has been inserting into the header of the file. The file is read out only if the fixed correlation holds.