A radio communication system including a base station and a mobile station, which perform radio communication. The mobile station includes: a receiver which receives a data signal transmitted from the base station; a controller which switches a number of group units and groups the data signal by the switched number of group units, according to one parameter or a combination of a plurality of parameters out of a type of a channel, a type of modulation scheme and encoding rate, an assigned resource amount, or a number of transmitting antennas of the mobile station, when an ACK signal or an NACK signal in response to the data signal is transmitted; and a transmitter which transmits the ACK signal or NACK signal in each group unit of the data signal. The base station includes: a transmitter which transmits the data signal; and a receiver which receives the ACK or NACK signals.