The upper limit of the number of the flow entries subjected to retrieval is increased by making it possible to retrieve the flow entries in plural switches for one communication data. Specifically, the control server registers flow entries by distributing them to the flow table in each switch belonging to a switch group. When acquiring the communication data, each switch checks whether the communication data is matched with any of the flow entries registered in the flow table. When the communication data is matched with a flow entry, the communication data is forwarded based on the forwarding procedure defined in the flow entry. When the communication data is not matched with any flow entry, the communication data is forwarded to “flow table unretrieved switch” belonging to the same switch group. When there is no flow table unretrieved switch, a query of the forwarding procedure is performed to the control server.