1. A network system comprising: a plurality of transfer devices to transfer packets; and a management server for managing virtual networks configured from the transfer devices, wherein: the management server includes: an interface to acquire network configuration information from the transfer devices, and a control unit to select a test target virtual network from the virtual networks; and the control unit: generates a plurality of virtual networks based on the configuration information; sorts the plurality of generated virtual networks into groups based on the transfer devices configuring the generated virtual networks and topology information for the transfer devices configuring the generated virtual networks; selects a virtual network from each of the groups, the selected virtual network being configured with a largest number of transfer devices; sets the selected virtual network as the test target virtual network in the group to which the selected virtual network belongs; calculates a redundant composition of the virtual network based on the configuration information; and sorts the virtual networks into groups based on the transfer devices that configure the transfer paths in the redundant composition and a physical topology table for the transfer devices that configure the transfer paths.