A relaying device for relaying data transferred from first storage unit to a second storage unit, is provided outside a range presumed to be affected by a disaster, when a disaster, such as an earthquake, has broken out in an installation site of a host of a normal system (host computer) and the first storage unit. Moreover, the relaying device is placed at such a location that the data transfer time between the first storage unit and the relaying device is shorter than the data transfer time in case the first and second storage units directly transfer data with each other. On receipt of data from the first storage unit, the relaying device notifies the first storage unit of the completion of reception of the data before completing transmission of the data to the second storage unit.