In a mobile communication system including a base station and a user terminal which communicate with each other according to FDD (frequency division duplex), an uplink frequency band and a downlink frequency band which are adjacent to each other are divided into plural time slots. The user terminal includes an interference detecting unit configured to detect interference in the downlink frequency band; and a time slot information generating unit configured to generate information about an available time slot in the downlink frequency band based on the detected interference. The base station includes a time slot allocating unit configured to allocate a time slot to be used in the downlink frequency band based on the information about the time slot received from the user terminal.