A radio base station that has a plurality of uplink component bands and a plurality of downlink component bands as a communication band for the radio base station, sets for each radio terminal a component band group formed with one of the plurality of uplink component bands in the communication band and an associated two or more of the plurality of downlink component bands, and is able to communicate with the radio terminal using the component band group, the radio base station being characterized by: a forming section that forms for each downlink component band a first channel signal that has a common search space that is common among a plurality of radio terminals and a user equipment specific search space that is individually assigned to each radio terminal; and an information size adjusting section that adjusts information sizes of uplink control information and downlink control information that are to be sent to an arbitrary transmission target terminal and that are included in the formed first channel signal, based on a size adjustment reference, in the common search space of a downlink additional component band that is included in the component band group, the downlink additional component band being not a basic component band that is a downlink component band in the component band group, and that is a downlink component band in which a broadcast channel signal that includes information about the uplink component band of the component band group is transmitted, using, as a common size adjustment reference among the plurality of radio terminals, a larger one of: the information size of downlink assignment control information determined from a bandwidth of the downlink additional component band; and the information size of uplink assignment control information determined from a bandwidth of the uplink component band that is associated with the downlink additional component band in the broadcast channel signal transmitted in the downlink additional component band, and in the user equipment specific search space of the downlink additional component band, using, as the size adjustment reference for each radio terminal, the larger one of the information size of downlink assignment control information determined from the bandwidth of the downlink additional component band and the information size of uplink assignment control information determined from the bandwidth of the uplink component band. a forming section that forms for each downlink component band a first channel signal that has a common search space that is common among a plurality of radio terminals and a user equipment specific search space that is individually assigned to each radio terminal; and an information size adjusting section that adjusts information sizes of uplink control information and downlink control information that are to be sent to an arbitrary transmission target terminal and that are included in the formed first channel signal, based on a size adjustment reference, in the common search space of a downlink additional component band that is included in the component band group, the downlink additional component band being not a basic component band that is a downlink component band in the component band group, and that is a downlink component band in which a broadcast channel signal that includes information about the uplink component band of the component band group is transmitted, using, as a common size adjustment reference among the plurality of radio terminals, a larger one of: the information size of downlink assignment control information determined from a bandwidth of the downlink additional component band; and the information size of uplink assignment control information determined from a bandwidth of the uplink component band that is associated with the downlink additional component band in the broadcast channel signal transmitted in the downlink additional component band, and in the user equipment specific search space of the downlink additional component band, using, as the size adjustment reference for each radio terminal, the larger one of the information size of downlink assignment control information determined from the bandwidth of the downlink additional component band and the information size of uplink assignment control information determined from the bandwidth of the uplink component band. the information size of downlink assignment control information determined from a bandwidth of the downlink additional component band; and the information size of uplink assignment control information determined from a bandwidth of the uplink component band that is associated with the downlink additional component band in the broadcast channel signal transmitted in the downlink additional component band, and in the user equipment specific search space of the downlink additional component band, using, as the size adjustment reference for each radio terminal, the larger one of the information size of downlink assignment control information determined from the bandwidth of the downlink additional component band and the information size of uplink assignment control information determined from the bandwidth of the uplink component band.