PURPOSE: To improve the safety of a file by securing such a constitution where the protection of the file cannot be invalidated even with the remodeling of software including an OS.

CONSTITUTION: A RAM 33 is provided together with a key collation part 32. When a data processor designates a piece of key information and a data block for access to them, the extent information including said data block is retrieved on the RAM 33. Then the part 32 collates the key information corresponding to said extent information with the key information received from the data processor. Then an access is allowed only when the coincidence is secured between both key information. Therefore it is impossible to invalidate the protection (collating conditions) of a file already registered just with the remodeling of software. In such a way, the file safety is improved.