A radio communication system includes a first base station that is identified by a first cell identity, a second base station that is identified by a second cell identity, and a user device that can perform radio communication with each of the base stations. The user device detects a first transmission timing based on a first synchronization signal from the first base station and establishes synchronization with the first base station. The first base station notifies the user device of synchronization state information related to a synchronization state with the second base station and of frequency information related to an identification signal frequency that is used by the second base station to transmit an identification signal. The user device, based on the synchronization state information, performs identifying processing, on the identification signal frequency indicated by the frequency information, to identify the second cell identity indicated by the identification signal.