When a mobile node makes handover, the mobile node issues a binding update message and sends the binding update message to a base transceiver station from which handover is made. When the base transceiver station from which handover is made sends radio data to the mobile node, it also transfers the same data to a base transceiver station to which handover is made, so that the base transceiver station to which handover is made also sends the radio data to the mobile node. As a result, the mobile node receives the radio data via an IP network from both the base transceiver station from which handover is made and the base transceiver station to which handover is made. Since the mobile node makes soft handover, the handover can be made without causing a communication short-break and a packet loss.