Provided are a radio terminal, a radio base station, a channel signal forming method, and a channel signal receiving method, by which the number of blinds in the processing for receiving allocation control information is reduced without causing disadvantage regarding the reception of a broadcast channel signal. In a base station (100), an information size adjustment unit (103) uses, in a common search space in a downlink additional unit band, the larger of the information size of downlink allocation control information determined from the bandwidth of a downlink additional unit band and the information size of uplink allocation control information determined from the bandwidth of an uplink unit band associated with the downlink additional unit band by a broadcast channel signal transmitted in the downlink additional unit band, as a size adjustment reference, and uses, in a UE specific search space, the larger of the information size of the downlink allocation control information determined from the bandwidth of the downlink additional unit band and the information size of the uplink allocation control information determined from the bandwidth of the uplink unit band of a unit band group, as the size adjustment reference.