There are provided a communication device, a base station, a communication system, and a communication method that make it possible to yield a diversity effect by cooperative relay without involvement of disproportionateness in data received by an eNB even when one of parties has unsuccessfully exchanged data. After received an ACK signal from a repeater 1, a repeater 2 which has transmitted a NACK signal in procedure 7 transmits data S2 to the eNB by use of its own resource, thereby making its own resource available for the repeater 1 (the repeater 2 does not use its own resource, and the repeater 1 uses the resource of the repeater 2). In procedures 8, the repeater 1 has received the NACK signal from the repeater 2 and subsequently transmits data S1 to the eNB by use of its own resource. Subsequently, the repeater 1 which has received the MACK signal in procedure 9 determines that the resource of the repeater 2 is available and transmits data P1 to the eNB by use of the resource of the repeater 2. In procedure 10, the repeater 1 transmits previously-generated data P2 to the eNB by use of its own resource.