PURPOSE: To ensure the sure file control despite the partial breakage of information as well as to minimize the memory control information, by using hierarchically the file control information to attain the decentralized storage of this information.

CONSTITUTION: The file devices 13 and 14 store a volume directory and a data directory which serve as the file control information respectively. The file control is performed by the volume directory for each optical disk, and the accesses are given to desired data from optical disks of optical disk devices 11 and 12 based on the data directory. If the device 14 has a fault and its data directory is broken, a CPU 10 specifies an optical disk by the volume directory and decides presence or absence of the data filed by the start and end data control information DS and DE. In addition, the optical disk is scanned through a final write address E and the corresponding data directly is retrieved. This data directory is stored in the device 13 and then used to give an access to a desired data part.