A radio communication system includes a radio network controller, base stations and mobile stations. The mobile station includes a response signal transmitter which is configured to transmit a response signal including a group ID identifying a multicast group A to a base station. The response signal responds to a control signal for the multicast group A which the mobile station is joining in. The base station includes a response signal transmitter configured to transmit at least one response signal to the radio network controller. The response signal is selected from at least one response signal transmitted from the mobile station joining in the same multicast group A.