According to this invention, there is provided a method applicable to the system which comprises a plurality of base stations each forming radio zones and storing a mobile terminal, and a gateway being connected to a packet communication network and relaying packet communication between the packet communication network and the mobile terminal. The method comprises buffering packets transmitted by a downlink to the mobile terminal in a packet buffer, requiring retransmission of unarrived packets in handover accompanied by movement among the radio zones, reading the packet required from the mobile terminal to retransmit the packet to the mobile terminal that is a request source, and assigning serial identification numbers to packets at least in a start to an end of the handover. When the identification numbers of the reached packets are discontinuous, the mobile terminal requires retransmission of packets needed to eliminate the discontinuity.