A mobile station device receiving at least one downlink control channel, the at least one downlink control channel including at least one control channel element among a plurality of control channel elements, each of the plurality of control channel elements including a plurality of time/frequency resources in a frequency band among a plurality of frequency bands, each of the plurality of frequency bands including a plurality of continuous subcarriers in downlink, the mobile station device acquiring, from the at least one downlink control channel, first information and second information as radio resource allocation information of a downlink shared data channel, the first information indicating one of the plurality of frequency bands, the second information indicating at least one resource block pair group, the mobile station device receiving the downlink shared data channel in the one of the plurality of frequency bands indicated by the first information and the at least one resource block pair group as indicated by the second information, the mobile station device comprising: a reception processing section configured to determine the number of the control channel elements included in a search space of one frequency band of the plurality of frequency bands, the determination being made in connection with the search space including the plurality of control channel elements in which the downlink control channel is decoded, the downlink control channel being allocated with a signal generated from third information and control information, the third information being obtained by performing an exclusive OR operation on a cyclic redundancy check, CRC, with a mobile station identification, the CRC being generated from the control information, and the control information being unique to the mobile station apparatus and including at least the radio resource allocation information, the downlink control channel including the radio resource allocation information, the downlink control channel being to be subject to a decoding and a detection, the determination further being made based on number of downlink control channels in the one frequency band of the plurality of frequency bands, decode the signal allocated in the downlink control channel, and detect the third information and the control information from a decoding result, the detected control information including the first information and the second information. a reception processing section configured to determine the number of the control channel elements included in a search space of one frequency band of the plurality of frequency bands, the determination being made in connection with the search space including the plurality of control channel elements in which the downlink control channel is decoded, the downlink control channel being allocated with a signal generated from third information and control information, the third information being obtained by performing an exclusive OR operation on a cyclic redundancy check, CRC, with a mobile station identification, the CRC being generated from the control information, and the control information being unique to the mobile station apparatus and including at least the radio resource allocation information, the downlink control channel including the radio resource allocation information, the downlink control channel being to be subject to a decoding and a detection, the determination further being made based on number of downlink control channels in the one frequency band of the plurality of frequency bands, decode the signal allocated in the downlink control channel, and detect the third information and the control information from a decoding result, the detected control information including the first information and the second information.