A synchronization confirmation method required for operation of a database is provided to a host device connected to a disk device which provides remote asynchronous data transfer. Immediately after commit of write data, an application of the host device issues a sync command which requires synchronization confirmation. A disk control device notifies the host device of a number concerning latest data received from the host device of a local side and a number concerning latest data received by a remote side. When the two numbers have coincided with each other, the host device judges the commit operation to have been completed.