A primary system includes a first node and a second node that backs up the first node. A secondary system includes a third node and a fourth node that backs up the third node. The first node in the primary system inserts, when transmitting data update information generated in response to a data update in the first node to the second node and the third node, one or a plurality of pieces of delimiter information indicating a boundary between update processing units, into both of transmit data. The fourth node in the secondary system specifies, based on the delimiter information, the data update information including update information whose process has progressed further from among the data update information obtained from the second node and the data update information obtained through the third node, and reflects the specified data update information to stored data of the fourth node.