Upon a receipt of an advance notice, an active system computer on which asynchronous replication is performed with a standby system computer stops a business application and transmits to the standby system computer transmission start information indicating start of data synchronization, data accumulated in a transmission queue 118, and transmission completion information indicating completion of the data synchronization in this order. The standby system computer generates and holds first reliability guarantee information indicating that the data received before the transmission start information upon a receipt of the transmission start information, and generates and holds second reliability guarantee information indicating that the data received before the transmission completion information is reliable data upon a receipt of the transmission completion information. Accordingly, while taking advantages of the asynchronous replication to the data replication, reliability of data backed up on the standby system computer is guaranteed upon a failure.