A correspondence table 0121 for recording the correspondence relationship of file titles, file IDs and managing IDs, hash tables 0122 for recording the correspondence relationships of hash values of files and managing IDs and a file memory 0123 for keeping managing IDs and files are provided. When registering files, a file with any registered same title file is managed as a separate file by giving a separate managing ID. File content comparison with registered files is done, and a file with any registered same content file is registered as single file even if it has been registered with a separate file ID. When calling out a file, the file to be called is specified by the combination of the file title and the file ID.