A radio base station improved in the efficiency of processing random accesses from mobile stations. The radio base station designates a random access signal when transmitting downlink data to a mobile station. The mobile station transmits the designated random access signal to the radio base station. The radio base station transmits, to the mobile station, timing adjustment information prepared based on the result of reception of the random access signal and allocation information about an uplink radio resource allocated to the mobile station. If there is control data to be transmitted to the radio base station, the mobile station transmits the control data by using the allocated uplink radio resource, and if there is no control data to be transmitted, the mobile station transmits a synchronization completion report prepared in reply to the timing adjustment information.