Each of the sites interconnected to one another through a wide area network includes at least one client, a switch device logically disposed between the client and at least one server including a storage device, for providing to the client a file access virtually unifying of a plurality of storage devices, and an analysis device connected to the switch device. The switch device takes log information on the file access from the client. The analysis device determines data to be migrated to the server in a site to which the client belongs or the server in a site in vicinity of the site to which the client belongs, and commands the switch device to migrate the data. The switch device migrates the data based on the command from the analysis device.