A backup-source storage system extracts, as a pattern, forwarding target data to a backup-destination storage system among data written to a storage apparatus through a volume. The backup-source storage system determines whether the extracted pattern matches with a registered pattern in the backup-source storage system. When a result of the determination is false, the backup-source storage system registers the extracted pattern and forwards the forwarding target data to the backup-destination storage system. When the result of the determination is true, the backup-source storage system does not forward the forwarding target data.