A lower-level master node sends, to a higher-level master node, a write set expanded in its own memory including a shadow copy of its own database and a heap tuple map, and the higher-level master node that received the write set verifies whether the update has already been executed and sends the record of this update to the lower-level master node as a transaction log, whereby the database can be updated efficiently and consistently from the lower-level master node to the higher-level master node, and from the higher-level master node to the lower-level master node under its control.