PURPOSE: To recover a data file even when readout is impossible by providing a writable memory and a controller for the memory, and always performing the backup of directory data on the memory in a fixed disk.

CONSTITUTION: When a central arithmetic processing unit 3 writes the data file generated in a main memory device 4 on a disk device 2, an instruction to read out the directory area 11 of a disk 7 to the main memory device is generated. Thereby, the directory data is read in. Next, it is judged whether or not the data file corresponding to a storage destination exists in read directory data. When no data exists, a null area in a data area 10 is searched, and the directory of the data file to be written from now is added on data read in the main memory device 4, and the instruction to write updated data on a disk controller 5 is generated.