PURPOSE: To attain retransmission in response to the quality of a line and to make the retransmission efficient by counting an error number in a received data and setting a maximum value of retransmission number of times smaller when the error number is more than a prescribed value.

CONSTITUTION: A data sender side protocol control section 21 sends a packet having a predetermined error control code, a reception side protocol control section 25 monitors a data error to count the error number. When the count is a prescribed threshold level or over. Since it means much deteriorated quality of the line and the possibility of sending a data correctly even with repeated retransmission is less, the retransmission number of times is set to a small value. When the count is less than the threshold level, since the possibility of sending the content of the data through the repeated transmission of the data is high, the retransmission number of times is set to a larger value. Thus, the efficient data transmission is attained.