a file management method according to the present invention is based upon a file management method of making a file from a client to a storage device redundant by a certain redundant number and storing the file in the storage device , and the file management method according to the present invention includes a first step of accepting an additional file from the client to the storage device , a second step of comparing capacity of the additional file and unused physical capacity of the storage device and a third step of changing the redundant number of the already stored file , increasing the unused physical capacity and storing the additional file in the storage device when the capacity of the additional file is larger than the unused physical capacity .