Disclosed is wireless communication base station equipment in which CCE allocation can be flexibly performed without collision of ACK/NACK signals between a plurality of unit bands, even when wideband transmission is performed exclusively on a downlink circuit. In this equipment, an allocation unit (105) sets up mutually different search spaces for each of a plurality of downlink unit bands, with respect to wireless communication terminal devices that communicate using a plurality of downlink unit bands, and allocates resource allocation information of downlink circuit data destined for the wireless communication terminal devices to CCEs in mutually different search spaces for each of the plurality of downlink unit bands, and an ACK/NACK reception unit (119); extracts a response signal in respect of the downlink circuit data from the uplink control channel associated with the CCE to which the resource allocation information of this downlink circuit data was allocated.