A remote file server managing a remote storage device and a local file server managing a local storage device are coupled to a communication network (e.g. the Internet). The local file server (A) replicates a file in the local storage device to the remote file server and, (B) if a first condition is satisfied, manages the replicated file which is the file from which the data is replicated as a migration target file. The local file server, even if the (B) is completed, does not delete the migration target file from the local storage device and, if the second condition is satisfied, deletes the migration target file from the local storage device.