Staging memory system and data migration method thereof

This invention provides a hierachical storage system with high performance, high capacity and low price, which includes a file server connected with a plurality of computers, and a second storage device connected with a first storage device, in which the second storage device has a second volume; the first storage device has a virtual volume virtualized with a first volume and the second volume as a self-device possessed volume; and the file server mounts the first volume as a first directory, mounts the virtual volume as a second directory, and copies predetermined files in the second directory into the first directory.