A base station controller selects codes corresponding to rate information included in a new starting call message from a mobile station, and further selects codes assignable to the mobile station from the selected codes as candidate codes. Then, the base station controller detects codes which meet both the characteristics of being at a higher level of the candidate codes in the tree structure and being assignable to other mobile stations, and further detects levels of the detected codes. Detected levels for all the candidate codes are compared to find a candidate code whose level is the lowest and to assign the candidate code to the mobile station sending the new staffing call message.