The present invention provides a radio base station which can equally assign a transmission opportunity based on the radio channel quality. A mobile station selection unit issues a request for a metric notification to a metric notification unit corresponding to a mobile station notified by a data flow control unit, and receives metrics. The metric notification unit periodically receives radio channel quality information from a channel quality notification unit of an uplink signal analysis unit, obtains through a rank calculation unit the rank at the time when the channel quality is preferably arranged in a channel quality buffer according to the radio channel quality information, and notifies the mobile station selection unit of the result as metrics. The mobile station selection unit selects a mobile station having the smallest metrics notified by the metric notification unit, assigns radio resources to the mobile station, and notifies the data flow control unit of the assignment.