In a configuration provided with routers 118-126and gateway device 130 connected above edge routers 102-116 provided in respective base stations in a hierarchical structure and configured to perform relaying of data to the base stations, a router 126 is specified as a relay router, which is a COR closer to a radio terminal unit out of CORs where all paths from an edge router 104 before movement of the radio terminal unit 100 and from each of all edge routers 102, 106 as destination candidates to CN 132 of a communication correspondent intersect. This makes it feasible to readily decrease the handover transaction time.