Under the environment that IP addresses of virtual machines overlap, it makes it possible to carry out a communication between the virtual machines and for a network manager to grasp in a communication flow condition in real time. Specifically, the physical server assigns a virtual machine ID to the virtual server under the management, and assigns a reception-side virtual machine ID and a transmission-side virtual machine ID to a bit space of a transmission packet in the form of TCP/IP packet, when a communicating between the virtual servers through a network. Then, the physical server transmits the transmission packet onto the network. A physical switch is provided on the network connecting between the physical servers, and identifies the transmission packet based on the virtual machine ID contained in a bit space other than data of the transmission packet under the environment that IP addresses of the virtual machines overlap on the network, and collects data showing a network condition.