A radio base station that is arranged to allocate component band groups on a per radio terminal basis and that is able to communicate with radio terminals using a component band groups, each component band group being formed with an uplink component band and a plurality of downlink component bands associated with the uplink component band, the radio base station comprising: a forming section adapted to form channel signals on a per downlink component band basis, each channel signal having a common region that is common between all radio terminals and a dedicated region that is allocated to an individual radio terminal on a dedicated basis, downlink assignment control information for an arbitrary target terminal being included in the common region and in the dedicated region, in all channel signals to be transmitted in a downlink component band allocated to the arbitrary target terminal, uplink assignment control information for the arbitrary target terminal being included in the dedicated region only in part of the channel signals, and included in the common region at least in the part of the channel signals; and an information size adjustment section adapted to adjust an information size of uplink assignment control information and downlink assignment control information for the arbitrary target terminal, included in the formed channel signals, based on a size adjustment reference, in all downlink component bands that are allocated to the arbitrary target terminal, in the common region, a larger one of a downlink assignment control information size determined from a bandwidth of a downlink component band in which a channel signal having the common region is sent, and an uplink assignment control information size determined from a bandwidth of an uplink component band associated with the downlink component band, being used as a size adjustment reference, in a dedicate region including the uplink assignment control information for the arbitrary target terminal, the larger one of the downlink assignment control information size determined from the bandwidth of the downlink component band in which the channel signal having the dedicated region is sent, and the uplink assignment control information size determined from the bandwidth of the uplink component band associated with the downlink component band, being used as the size adjustment reference, in a dedicate region not including the uplink assignment control information for the arbitrary target terminal, the downlink assignment control information size determined from the bandwidth of the downlink component band in which the channel signal having the dedicated region is sent, being used as the size adjustment reference. a forming section adapted to form channel signals on a per downlink component band basis, each channel signal having a common region that is common between all radio terminals and a dedicated region that is allocated to an individual radio terminal on a dedicated basis, downlink assignment control information for an arbitrary target terminal being included in the common region and in the dedicated region, in all channel signals to be transmitted in a downlink component band allocated to the arbitrary target terminal, uplink assignment control information for the arbitrary target terminal being included in the dedicated region only in part of the channel signals, and included in the common region at least in the part of the channel signals; and an information size adjustment section adapted to adjust an information size of uplink assignment control information and downlink assignment control information for the arbitrary target terminal, included in the formed channel signals, based on a size adjustment reference, in all downlink component bands that are allocated to the arbitrary target terminal, in the common region, a larger one of a downlink assignment control information size determined from a bandwidth of a downlink component band in which a channel signal having the common region is sent, and an uplink assignment control information size determined from a bandwidth of an uplink component band associated with the downlink component band, being used as a size adjustment reference, in a dedicate region including the uplink assignment control information for the arbitrary target terminal, the larger one of the downlink assignment control information size determined from the bandwidth of the downlink component band in which the channel signal having the dedicated region is sent, and the uplink assignment control information size determined from the bandwidth of the uplink component band associated with the downlink component band, being used as the size adjustment reference, in a dedicate region not including the uplink assignment control information for the arbitrary target terminal, the downlink assignment control information size determined from the bandwidth of the downlink component band in which the channel signal having the dedicated region is sent, being used as the size adjustment reference.