A receiving side data communication device calculates, at prescribed times, an actual measured RTT, which is the time required for data to travel to a transmission side data communication device and back, a data transfer rate for the data transfer, and a data loss ratio, which is the ratio of the number of packets not received by the receiving side data communication device to the number of packets sent from the transmission side data communication device. The receiving side data communication device uses the most recent data transfer rate and the data loss ratio to calculate a transfer score according to a prescribed calculation formula, and increases or decreases the reference RTT in accordance with the change in the transfer score.