The track information table has: decrypting information corresponding to each track; file pointer information showing one of a plurality of files with headers; and index information showing positions in the files. A function for adding the header to the track of the index information is provided. The decrypting information corresponding to the selected track, the file pointer information, and the index information are read out from the track information table. The header of the file with the header is read out on the basis of the file pointer information. Position information in the file corresponding to the index information is detected from the header. The latter half data of the file is read out on the basis of the position information and added to the track in which coupling has been instructed, thereby newly forming a file with a header.