In a network system, each server is assigned to at least one relay processing devices. A plurality of switches are arranged in a network and a controller controls the switches. Specifically, the controller designs, in response to a request from a request source switch, a route of a flow from the request source switch to a destination address. Here, the controller refers to assignment information indicating an assignment relationship between servers and relay processing devices to select, as a target, any one relay processing device assigned to a server specified by the destination address, and designs the route so as to include the target. Then, the controller instructs each switch on the designed route to set the flow table such that a packet is forwarded along the designed route.