A community control server stores in a community defining table, data of a correspondence between Interior Gateway Protocol (IGP) routers that are connected to a community and another community. The community control server collects prefix data that is included in routing information that is received by the IGP routers and identification data of the IGP routers. Based on the community defining table and the identification data, the community control server detects a community corresponding to a prefix. Upon receiving data of whether to transmit the routing information in the form of the permission to distribute them between communities, based on data of the community the community control server determines the prefix corresponding to the source community and transmits the prefix data to the IGP routers.