1. A file system migration method used in a storage system that includes a plurality of servers and a disk subsystem coupled to the plurality of servers, having all or a part of at least one file system stored in a logical device of the disk subsystem and an administrative computer, and providing a file name space to a client computer by the plurality of servers the file system migration method comprising: managing a first information indicating a relationship between a plurality of path names of the file name space and a plurality of server identifiers, referencing, in response to an instruction to execute migration of the file system from a first server to a second server out of the plurality of servers, a second information indicating a relation between the logical device of the disk subsystem and a logical unit used for the plurality of servers to recognize the logical device; sending an instruction to update the relation between the logical device and the logical unit to the second server; causing the second server to newly recognize the logical unit, in response to the instruction to update, and after recognition of the logical unit, updating the first information to indicate that a path name previously managed by the first server is related to the second server, to thereby migrate all or at least part of the file system without executing a data copy operation.