A file storage system comprising: a controller for managing a plurality of files each having management information and actual data; a storage medium constituting a volume for storing the plurality of files, wherein the volume includes a first directory and a second directory, the first directory storing a first file and a second file; wherein the controller is configured to: create a third file in the second directory, and set up the management information of the third file so that an actual data storage area of the second file is shown as an actual data storage of the third file; set up the management information of the second file to refer to the third file when the second file receives a read access from the computer, and delete the management information showing the actual data storage area of the second file from the management information of the second file; compare a size of the actual data of the first file with a size of the actual data of the third file; if, as a result of the size comparison, the data sizes are identical, compare a binary of the actual data of the first file with a binary of the actual data of the third file; and if, as a result of the binary comparison, the binaries are identical, set up the management information of the first file to refer to the third file so that the third file is referenced when the first file receives a read access from the computer, and delete the actual data of the first file, wherein the second directory is an index directory; wherein a third directory is provided in the second directory; wherein a size of the third file is set as a directory name of the third directory; and wherein in order to compare the size of the actual data of the first file with the size of the actual data of the third file, the controller is configured to compare the size of the actual data of the first file with the size denoted in the directory name of the third directory instead of the size of the actual data of the third file. a controller for managing a plurality of files each having management information and actual data; a storage medium constituting a volume for storing the plurality of files, wherein the volume includes a first directory and a second directory, the first directory storing a first file and a second file; wherein the controller is configured to: create a third file in the second directory, and set up the management information of the third file so that an actual data storage area of the second file is shown as an actual data storage of the third file; set up the management information of the second file to refer to the third file when the second file receives a read access from the computer, and delete the management information showing the actual data storage area of the second file from the management information of the second file; compare a size of the actual data of the first file with a size of the actual data of the third file; if, as a result of the size comparison, the data sizes are identical, compare a binary of the actual data of the first file with a binary of the actual data of the third file; and if, as a result of the binary comparison, the binaries are identical, set up the management information of the first file to refer to the third file so that the third file is referenced when the first file receives a read access from the computer, and delete the actual data of the first file, wherein the second directory is an index directory; wherein a third directory is provided in the second directory; wherein a size of the third file is set as a directory name of the third directory; and wherein in order to compare the size of the actual data of the first file with the size of the actual data of the third file, the controller is configured to compare the size of the actual data of the first file with the size denoted in the directory name of the third directory instead of the size of the actual data of the third file. create a third file in the second directory, and set up the management information of the third file so that an actual data storage area of the second file is shown as an actual data storage of the third file; set up the management information of the second file to refer to the third file when the second file receives a read access from the computer, and delete the management information showing the actual data storage area of the second file from the management information of the second file; compare a size of the actual data of the first file with a size of the actual data of the third file; if, as a result of the size comparison, the data sizes are identical, compare a binary of the actual data of the first file with a binary of the actual data of the third file; and if, as a result of the binary comparison, the binaries are identical, set up the management information of the first file to refer to the third file so that the third file is referenced when the first file receives a read access from the computer, and delete the actual data of the first file, wherein the second directory is an index directory; wherein a third directory is provided in the second directory; wherein a size of the third file is set as a directory name of the third directory; and wherein in order to compare the size of the actual data of the first file with the size of the actual data of the third file, the controller is configured to compare the size of the actual data of the first file with the size denoted in the directory name of the third directory instead of the size of the actual data of the third file.