In the present invention, if a transmitting device receives an activation command for a packet duplication function of a PDCP entity for a radio bearer (RB), the PDCP entity submits a PDCP protocol data unit (PDU) to each of multiple lower layer entities associated with the PDCP entity, and each of the multiple lower layer entities transmits the PDCP PDU. If the transmitting device receives a deactivation command for the packet duplication function, the PDCP entity submits the PDCP PDU to a single one of the multiple lower layer entities, and the single one of the multiple lower layer entities transmits the PDCP PDU.