User equipment for communicating with a base station (eNB) in a radio communication system supporting carrier aggregation, the user equipment comprising: a receiver configured to receive, from the base station (eNB), a capability information request signal including information indicating a plurality of bands supported by the base station (eNB); a generator configured to generate band combination information indicating a band combination capable of being used for the carrier aggregation in the user equipment among the plurality of bands supported by the base station (eNB), and a parameter configured for the band combination; and a transmitter configured to transmit the generated band combination information to the base station (eNB), wherein the generator is configured to generate the band combination information including information indicating a highest band combination having a largest number of component carriers, among the plurality of bands supported by the base station (eNB), and the user equipment characterised by being configured so that, upon being instructed by the base station (eNB) to report, among all band combinations with numbers of component carriers that are less than a number of component carriers of a predetermined band combination, a band combination including a parameter that is different from a parameter corresponding to the predetermined band combination, the generator generates, instead of the information indicating the highest band combination information, the band combination information including information indicating the band combination including the parameter that is different from the parameter corresponding to the predetermined band combination, among the all band combinations with the numbers of component carriers that are less than the number of component carriers of the predetermined band combination. a receiver configured to receive, from the base station (eNB), a capability information request signal including information indicating a plurality of bands supported by the base station (eNB); a generator configured to generate band combination information indicating a band combination capable of being used for the carrier aggregation in the user equipment among the plurality of bands supported by the base station (eNB), and a parameter configured for the band combination; and a transmitter configured to transmit the generated band combination information to the base station (eNB), wherein the generator is configured to generate the band combination information including information indicating a highest band combination having a largest number of component carriers, among the plurality of bands supported by the base station (eNB), and the user equipment characterised by being configured so that, upon being instructed by the base station (eNB) to report, among all band combinations with numbers of component carriers that are less than a number of component carriers of a predetermined band combination, a band combination including a parameter that is different from a parameter corresponding to the predetermined band combination, the generator generates, instead of the information indicating the highest band combination information, the band combination information including information indicating the band combination including the parameter that is different from the parameter corresponding to the predetermined band combination, among the all band combinations with the numbers of component carriers that are less than the number of component carriers of the predetermined band combination.