A radio communication system implementing Carrier Aggregation including a communication terminal operating in a first and a second frequency, and a base station operating in the second frequency, wherein the base station comprises a controlling unit that is configured to: receive terminal capability information from a communication terminal; determine whether the communication terminal is capable of using an unlicensed band by using the terminal capability information; control whether the first frequency of which use is unlicensed is to be used for first communication of the communication terminal; receive a measured radio link quality to and from a second base station from the communication terminal; select a cell of the second base station as a cell to perform communication using the first frequency based on the received measured radio link quality; transmit system information received from the second base station to the communication terminal; perform second communication with the communication terminal by using the second frequency of which use is licensed; and when the first frequency is to be used for the first communication of the communication terminal, use the second frequency to notify the communication terminal to use the first frequency for the first communication, and the communication terminal comprises a communicating unit (31, 33) that is configured to: transmit the terminal capability information including unlicensed band use capability/incapability information to the base station; measure the radio link quality to and from the second base station; transmit the measured radio link quality to the base station; receive the system information of the second base station from the base station; and when notified of using the first frequency by the controlling unit, perform the first communication with the second base station by using the first frequency of which use is unlicensed and perform the second communication with the base station by using the second frequency of which use is licensed. the base station comprises a controlling unit that is configured to: receive terminal capability information from a communication terminal; determine whether the communication terminal is capable of using an unlicensed band by using the terminal capability information; control whether the first frequency of which use is unlicensed is to be used for first communication of the communication terminal; receive a measured radio link quality to and from a second base station from the communication terminal; select a cell of the second base station as a cell to perform communication using the first frequency based on the received measured radio link quality; transmit system information received from the second base station to the communication terminal; perform second communication with the communication terminal by using the second frequency of which use is licensed; and when the first frequency is to be used for the first communication of the communication terminal, use the second frequency to notify the communication terminal to use the first frequency for the first communication, and receive terminal capability information from a communication terminal; determine whether the communication terminal is capable of using an unlicensed band by using the terminal capability information; control whether the first frequency of which use is unlicensed is to be used for first communication of the communication terminal; receive a measured radio link quality to and from a second base station from the communication terminal; select a cell of the second base station as a cell to perform communication using the first frequency based on the received measured radio link quality; transmit system information received from the second base station to the communication terminal; perform second communication with the communication terminal by using the second frequency of which use is licensed; and when the first frequency is to be used for the first communication of the communication terminal, use the second frequency to notify the communication terminal to use the first frequency for the first communication, and the communication terminal comprises a communicating unit (31, 33) that is configured to: transmit the terminal capability information including unlicensed band use capability/incapability information to the base station; measure the radio link quality to and from the second base station; transmit the measured radio link quality to the base station; receive the system information of the second base station from the base station; and when notified of using the first frequency by the controlling unit, perform the first communication with the second base station by using the first frequency of which use is unlicensed and perform the second communication with the base station by using the second frequency of which use is licensed. transmit the terminal capability information including unlicensed band use capability/incapability information to the base station; measure the radio link quality to and from the second base station; transmit the measured radio link quality to the base station; receive the system information of the second base station from the base station; and when notified of using the first frequency by the controlling unit, perform the first communication with the second base station by using the first frequency of which use is unlicensed and perform the second communication with the base station by using the second frequency of which use is licensed.