A communication section controls the transmission directivity of a plurality of antennas, based on a known signal transmitted from a communication terminal. A radio resource allocating section allocates a downlink radio resource and allocates an uplink radio resource. A grouping processing section for classifying classifies a plurality of communication terminals with which the communication section communicates into a first terminal group executing an application with high communication quality of transfer data and a second terminal group executing an application with low communication quality of transfer data. A unit period is determined in which an uplink radio resource for a known signal usable for the transmission of the known signal by a communication terminal and a downlink radio resource appear. A correspondence between the downlink radio resource and the uplink radio resource for the known signal is defined for null steering based on the known signal in the unit period.