To obtain a method capable of coping with rapid throughput variation by allowing a transmission side to divide a reception confirming data quantity on a receiving side by a time difference between first and second receiving times so as to calculate an estimated throughput to calculate a transfer data quantity to send out from a transmission side by this estimated throughput.

After executing calculation processing of a minimum reception and response detecting time RIT, estimated throughput calculating processing is executed. First, an (n)-DATA transmission history 14 and an (n)-ACK reception history 15 received this time are referred to calculate the reception confirming data quantity. Next, the receiving time of (n)-ACK received this time from the history 15 is obtained to obtain the receiving time of (n)-ACK received last time. Then the reception confirming data quantity is divided by the difference between the receiving time of this time and that of last time to calculate a newest estimated throughput by (n)-ACK received this time to store as an estimated throughput 18.