On the transmission side, the RLC PDUs divided from the RLC SDU are transmitted after retransmission identification information is attached thereto. On the reception side, the retransmission identification information attached to each received RLC PDU is referred to, and only the RLC PDUs having an identical retransmission identification information to a set assumed value are stored in a buffer memory, and the RLC PDUs having the identical retransmission identification information are assembled into an RLC SDU.