In cooperation between each data center and a WAN, virtual machine migration is carried out without interruption in processing so as to enable effective power-saving implementation, load distribution, or fault countermeasure processing. Each node located at a boundary point between the WAN and another network is provided with a network address translation (NAT) function that can be set dynamically to avoid address duplication due to virtual machine migration. Alternatively, each node included in the WAN is provided with a network virtualization function; and there are implemented a virtual network connected to a data center for including a virtual machine before migration, and a virtual network connected to a data center for including the virtual machine after migration, thereby allowing coexistent provision of identical addresses. Thus, the need for changing network routing information at the time of virtual machine migration can be eliminated, and a setting change for migration accomplished quickly.