A radio base station 10 communicates with a mobile station by use of an OFDM scheme while using a signal as a transmission unit, the signal including an effective period for transmitting a data signal and a redundancy period for transmitting redundancy data based on the data signal transmitted during the effective period. The radio base station 10 measures an interference level of a signal transmitted from the mobile station and then changes the length of the redundancy period of the signal transmitted via a communication channel used for communications with the mobile station.