Embodiments of the present invention make it possible to reduce loss and delay of downlink user data transmitted at regular time intervals in a mobile communication system employing a hard handover method, and make it possible to easily perform high-quality handover solely by a handover-starting base station. A base station for transmitting downlink user data at regular time intervals to a mobile station is disclosed. The base station is configured to transmit a handover-command signal immediately after transmitting the downlink user data to the mobile station.