In a radio communication system, a user terminal transmits a reservation packet to a base station using a common control channel (1), and the base station assigns an uplink data channel to the user terminal according to the reservation packet, and assigns a first shared control channel (2b) to the user terminal in association with the uplink data channel, and the mobile station transmits a control signal to the base station using the associated uplink shared control channel. According to status of radio communication, the base station may assign beforehand a second uplink shared control channel (2a) for which occupation by the user terminal is permitted only while the user terminal is performing communication.