A radio base station configured to assign control channel elements including a plurality of continuous resource element groups in an available radio resource to a physical downlink control channel, the radio base station comprising: an aggregation level determination unit configured to determine an aggregation level indicating the number of continuous control channel elements to be assigned to a physical downlink control channel addressed to a mobile station, based on reception quality notified from the mobile station; a control channel element assignment unit configured to assign a plurality of continuous control channel elements to the physical downlink control channel addressed to the mobile station, based on the determined aggregation level; and a resource assignment unit configured to assign resource element groups constituting the assigned control channel elements as a resource for the physical downlink control channel addressed to the mobile station, wherein the control channel element assignment unit is configured to assign a plurality of continuous control channel elements to the physical downlink control channel addressed to the mobile station based on an aggregation level changed by the aggregation level determination unit, in response to failing to assign control channel elements to the physical downlink control channel addressed to the mobile station, wherein the control channel element assignment unit is configured to sequentially assign control channel elements to a physical downlink control channel addressed to a first mobile station with highest priority and thereafter assign control channel elements to a physical downlink control channel addressed to at least one second mobile station other than the first mobile station; said radio base station being characterized in that: the control channel element assignment unit is configured to set a pattern defining which of possible control channel elements are to be assigned to the first mobile station, said pattern being a pattern for which the number of mobile stations not being assigned control channel elements to a physical downlink control channel is the smallest, and assign control channel elements to physical downlink control channels addressed to the first and to the at least one second mobile stations by employing said pattern. an aggregation level determination unit configured to determine an aggregation level indicating the number of continuous control channel elements to be assigned to a physical downlink control channel addressed to a mobile station, based on reception quality notified from the mobile station; a control channel element assignment unit configured to assign a plurality of continuous control channel elements to the physical downlink control channel addressed to the mobile station, based on the determined aggregation level; and a resource assignment unit configured to assign resource element groups constituting the assigned control channel elements as a resource for the physical downlink control channel addressed to the mobile station, wherein the control channel element assignment unit is configured to assign a plurality of continuous control channel elements to the physical downlink control channel addressed to the mobile station based on an aggregation level changed by the aggregation level determination unit, in response to failing to assign control channel elements to the physical downlink control channel addressed to the mobile station, wherein the control channel element assignment unit is configured to sequentially assign control channel elements to a physical downlink control channel addressed to a first mobile station with highest priority and thereafter assign control channel elements to a physical downlink control channel addressed to at least one second mobile station other than the first mobile station; said radio base station being characterized in that: the control channel element assignment unit is configured to set a pattern defining which of possible control channel elements are to be assigned to the first mobile station, said pattern being a pattern for which the number of mobile stations not being assigned control channel elements to a physical downlink control channel is the smallest, and assign control channel elements to physical downlink control channels addressed to the first and to the at least one second mobile stations by employing said pattern. the control channel element assignment unit is configured to set a pattern defining which of possible control channel elements are to be assigned to the first mobile station, said pattern being a pattern for which the number of mobile stations not being assigned control channel elements to a physical downlink control channel is the smallest, and assign control channel elements to physical downlink control channels addressed to the first and to the at least one second mobile stations by employing said pattern.