To reduce the processing load such as channel assignment and an authentication process in a plurality of radio base stations and communication terminals, and quickly and efficiently assign channels to the communication channels. The channel assigning method by a radio base station group, includes determining assignment of a communication channel between a communication terminal entering a radio area of the radio station group and all radio base stations within the radio station group by a base station in the radio base station group. Channel assigning information common to the radio base station group is updated as to the determined communication channel and a communication channel is assigned to the communication terminal with reference to the channel assigning information by each radio base station in the radio base station group other than the radio base station.