A base station in a master cell group configured to establish a bearer to a mobile station together with a base station in a secondary cell group for communication includes a data transfer instruction unit configured to transmit to the base station in the secondary cell group an instruction on whether to transfer downlink data in the base station in the secondary cell group to the base station in the master cell group upon deletion or modification of the secondary cell group or upon deletion of the bearer in the secondary cell group; a reception unit configured to receive the downlink data transferred from the base station in the secondary cell group according to the instruction; and a transmission unit configured to transmit the received downlink data to the mobile station.