In a storage system in which plural end servers 104, intermediate servers 103 and clients 101 are connected to each other via a network 105, when a migration-source end server 104 transfers file-level data to a migration-destination end server 104 via the intermediate server 103, the migration-source end server 104 also transfers data in a differential volume simultaneously, and the migration-destination end sever 104 reconstructs data in a snapshot volume based on the transferred data and transfers response information regarding the reconstructed data to the client 101.