1. A network system comprising: a plurality of transfer devices to transfer packets; and a management server for managing the network configured from the applicable transfer devices, wherein the management server includes: an interface to acquire network configuration information from the transfer devices, a memory unit to store the configuration information, and a control unit to generate virtual network information based on the configuration information; sort the virtual networks into groups based on the transfer devices configuring the applicable virtual networks, and topology information for the transfer devices configuring the virtual applicable networks; select one virtual network from each of the groups; and set the selected virtual network as the test target virtual network in the group that the applicable virtual network belongs.