In file de-duplication using hash value comparison, hash values of all target files must be calculated and actual data of all files must be read for hash value calculation, so that the processing time was long. The present invention provides a file storage system comprising a controller and a volume storing a plurality of files, the volume including a first directory storing a first file and a second file and a second directory storing a third file being created, wherein the controller migrates actual data of the second file to the third file, sets up a management information of the second file so that the third file is referred to when the second file is read, and if the sizes of actual data of the first file and the actual data of the third file are identical and the binaries of the actual data of the first file and the actual data of the third file are identical, sets up a management information of the first file to refer to the third file when reading the first file.