In an open flow network system in which there are a plurality of networks, in each of which one OFC (OpenFlow Controller) manages a plurality of OFSs (OpenFlow Switches), and a large network is formed by a combination of them, registration of a flow entry which forms a closed loop is prevented without a component and a procedure for cooperation of the plurality of OFCs. Specifically, after route calculation, the OFC temporarily registers an flow entry of a calculation result on a flow table of the OFS as the flow entry for a test. After that, the OFS flows the test packet according to the temporarily registered flow entry. When the test packet is routed without forming the closed loop, the OFS detects it and normally registers the flow entry for a first time. Detecting that the test packet forms a closed loop, the OFS deletes a temporarily registering flow entry and returns an error to the OFC.