A storage apparatus installed in a first site generates a snapshot of a copy source logical device in an intermediate logical device. Next, an external storage apparatus including a logical device corresponding to the intermediate logical device is disconnected from the first site and connected to the second site. A storage apparatus in the second site is made to recognize the logical device in the external storage device as a logical device of a remote copy target. Next, the storage apparatus instructs the second site about writing in the copy target logical device through a global network on the basis of the difference data managed with the bit map.