A wireless communication system comprising a mobile station apparatus and a base station apparatus which transmits downlink control information to said mobile station apparatus, wherein said base station apparatus is configured to: in a case that payload sizes of respective downlink control information of a physical downlink control channel candidate of a common search space and a physical downlink control channel candidate of a user equipment-specific search space are the same as each other, and said physical downlink control channel candidate of said common search space and said physical downlink control channel candidate of said user equipment-specific search space are comprised of same control channel elements, and wherein said base station apparatus is configured to indicate to said mobile station apparatus whether or not a carrier indicator is included in downlink control information which can be transmitted in said user equipment-specific search space by said base station apparatus, for said physical downlink control channel candidate of said common search space and said physical downlink control channel candidate of said user equipment-specific search space, allocate said downlink control information only in a physical downlink control channel candidate of said common search space and transmit said downlink control information, and said mobile station apparatus is configured to: in said case, for said physical downlink control channel candidate of said common search space and said physical downlink control channel candidate of said user equipment-specific search space, perform decoding of said downlink control information only in said physical downlink control channel candidate of said common search space, wherein the user equipment-specific search space is defined on the basis of C-RNTI for said mobile station apparatus. in a case that payload sizes of respective downlink control information of a physical downlink control channel candidate of a common search space and a physical downlink control channel candidate of a user equipment-specific search space are the same as each other, and said physical downlink control channel candidate of said common search space and said physical downlink control channel candidate of said user equipment-specific search space are comprised of same control channel elements, and wherein said base station apparatus is configured to indicate to said mobile station apparatus whether or not a carrier indicator is included in downlink control information which can be transmitted in said user equipment-specific search space by said base station apparatus, for said physical downlink control channel candidate of said common search space and said physical downlink control channel candidate of said user equipment-specific search space, allocate said downlink control information only in a physical downlink control channel candidate of said common search space and transmit said downlink control information, and said mobile station apparatus is configured to: in said case, for said physical downlink control channel candidate of said common search space and said physical downlink control channel candidate of said user equipment-specific search space, perform decoding of said downlink control information only in said physical downlink control channel candidate of said common search space, wherein the user equipment-specific search space is defined on the basis of C-RNTI for said mobile station apparatus. in said case, for said physical downlink control channel candidate of said common search space and said physical downlink control channel candidate of said user equipment-specific search space, perform decoding of said downlink control information only in said physical downlink control channel candidate of said common search space, wherein the user equipment-specific search space is defined on the basis of C-RNTI for said mobile station apparatus.