In a cluster type NAS, it is necessary to copy file data between disk devices of servers upon migration of a file system or a part of the file system. A server includes a control table for managing a file system or a part of the file system, and a mapping table for managing a relation between a logical device of a disk subsystem and a logical unit recognized by a server, and in response to an instruction to execute migration of the file system from a first server to a second server, the mapping table is referenced to send an instruction to update the relation between the logical device and the logical unit to the second server, and the second server newly recognizes a logical unit and updates a management server identifier of the control table.