As a method for migrating data of a volume adopting a snapshot function to a new storage system, in order to perform migration without depending on a method for compressing snapshot data of a migration source storage system, and without stopping transmission and reception of data between the host computer and the storage system, at first, after migrating data of a volume being the source of snapshot (PVOL), migration is performed sequentially from newer generations. At this time, migration target data of each SVOL is all the data within the migration source storage system. The SVOL data copied to a migration destination storage is compared with one-generation-newer SVOL data within the migration destination storage system, and based on the comparison result, a difference management information is created. If there is difference, a VOL allocation management table is updated, and difference data is stored in the area allocated within the pool.