In a method for transferring packet data remaining in a first SeNB to a second SeNB via an MeNB, use of a communication line is inefficient. A base station in a communication system that includes a master cell base station, a secondary cell base station, a core network, and a terminal device, wherein: the base station has a reception unit for receiving, from the master cell base station, downlink user data received from the core network by the master cell base station, and a transmission unit for transmitting the downlink user data received to the terminal device; the reception unit receives, from the master cell base station, switching information indicating that the base station from which the master cell base station receives downlink user data is to be switched; and the transmission unit transmits remaining data to the secondary base station when the reception unit has received the switching information.