A data backup system is provided with: a plurality of devices for main site storage, one device for relay site storage, and one or more devices for backup site storage. While the main site is operating normally, the relay site storage implements synchronized replication of data held by the main site storage and data held by the relay site storage. After the main site has stopped operating normally, the data held by the relay site storage and the data held by the backup site storage are subjected to consistency ensuring replication.