The present invention enables dynamic route switching according to link quality, while maintaining the quality of a plurality of routes. According to quality information received from a second network control entity, if the control unit of a network management station has determined that the quality of communication over a particular link has degraded, the control unit assigns a link whose current throughput satisfies a bandwidth required for a service defined in the service contract and whose response time satisfies minimum response time defined in the service contract to each service in descending order of priority.