This invention can make data communications at an optimal transfer rate on the basis of the unarrived data volume on a network between two end terminals. For this purpose, a transmitting terminal (1-1) adds sequence number information to data generated by a data generator (1-11), and transmits the data to a receiving terminal (1-2) via a data transmitter (1-12). Since the receiving terminal (1-2) transmits data including the sequence number in the received data, the transmitting terminal determines that data (buffer capacity) corresponding to the difference between the current sequence number and the received sequence number remain on the network and calculates that volume using a network buffer data volume calculator (1-14) The transmitting terminal determines the transmission rate on the basis of the calculation result, and controls the data transmitter (1-12) to transfer data at that transmission rate.