A variety of radio communication systems that operate in accordance with 3GPP (Third Generation Partnership Project) protocols are used in the art. In a radio communication system that operates in accordance with LTE/SAE (Long Term Evolution/System Architecture Evolution) protocols, which are parts of 3GPP protocols, an X2 interface is specified as an interface that connects base stations (eNB). Each base station communicates with another base station through an X2 interface. An X2 interface is specified to connect base stations that are of equal rank.