To control a radio link capacity in a backhaul link optimally, and to enhance a throughput in an access link, in the radio communications system using the relay transmission technology. A relay frequency allocation method according to the present invention includes: determining whether or not carrier aggregation is to be applied in a backhaul link from a radio base station apparatus to a mobile terminal device under the command of the radio base station apparatus, and/or an access link from a radio relay station apparatus to a mobile terminal device under the command of the radio relay station apparatus; allocating a frequency band for transmitting a downlink signal in the backhaul link and/or the access link, when applying the carrier aggregation; and transmitting the downlink signal with the allocated frequency band in the backhaul link and/or the access link.