A radio communication system, that includes, a source base station; a target base station; and a mobile station in which the mobile station includes a receiving means for receiving first PDCP (Packet Data Convergence Protocol) PDU (Protocol Data Unit) and first sequence number from the source base station and a receiving means for receiving second PDCP PDU from the target base station, in which the second PDCP PDU being created using second sequence number and PDCP (Packet Data Convergence Protocol) SDU (Service Data Unit) that are transferred from the source base station to the target base station; a storage means for storing PDCP SDU corresponding to the first PDCP PDU and PDCP SDU corresponding to the second PDCP PDU; and a reordering means for performing in order delivery of the stored PDCP SDUs based on the first sequence number and the second sequence number.