In a data transmission device operable in accordance with a protocol such that the same transmission data packet is automatically repeated on nonreception of the transmission data packet at a reception end, a repetition number of the same transmission data packet is measured in a protocol interface circuit together with a selected one of a packet size and a data size carried by the transmission data packet. The repetition number and the selected size are supplied to first and second conversion tables to be converted into first and second factors which are related to a bit error rate of the transmission data packet on a transmission line and a correction factor of the first factor, respectively. The first and the second factors are added to each other to be averaged into an estimation factor of the bit error rate in consideration of preceding data packets. The estimation factor is converted by the use of a third conversion table into an optimum size of either a packet size or a data size. Consequently, transmission efficiency is improved by transmitting each transmission packet with an optimum size.