PURPOSE: To improve the efficiency of a receiving buffer and the throughput of a circuit, by transmitting the receivable number of words of the receiving buffer accompanying an answer field from a receiving station, and by transmitting only data, whose number will not exceed the receivable number, from a transmitting station.

CONSTITUTION: In the receiving station, the receivable number of words, which is the number of remaining words of receiving buffer 14 is transmitted to the transmitting station accompanying the answer field in a transmission frame on a circuit simultaneously with the reception of data from the transmitting station by receiving buffer 14. In the transmitting station, on the other hand, the receivable number of words sent from the receiving station is fetched into answer data count counter- register 20, where it is compared 19 with the content of transmitting-data word number register 18 preset, and comparator 22 compares smaller one with the content of transmitting-data counter 21; and then, data of words whose number is below the transmitted receivable number of words are transmitted to the receiving station, and an end code is transmitted after error control information. Consequently, the efficiency of the receiving buffer and the throughput of the circuit are improved.