An object is to transmit data such that the transmission speed is not reduced even in a transmission route with a comparatively high percentage of packet loss.The transmitter 1 comprises: a data transmission unit 11 which transmits data packets 50 comprising data, a data sequence number indicating the order of the data, and a transmission sequence number indicating the transmission order, and which receives acknowledgement packets 60 comprising the next data sequence number, a bit string indicating the reception status, and a received transmission sequence number; a transmission history memory unit 14 which memorizes the transmission history of the data packets transmitted by the data transmission unit 11; and a retransmission control unit 12 which refers to the transmission history memory unit 14 and specifies the data sequence numbers requiring retransmission based on the received acknowledgement packets 60, and which directs the data transmission unit 11 to retransmit. The receiver 2 comprises a data reception unit 21 which receives data packets 50 transmitted from the transmitter 1, and which produces and transmits to the transmitter 1 acknowledgement packets 60 corresponding to these packets.