PURPOSE: To prevent a transmission error, to save a man-hour and communication cost and to avoid an undesired line occupancy by allowing a sender side to recognize the availability of reception and a receptible capacity or the like of a receiver side in advance before the sender side acquires a line.

CONSTITUTION: A reception request command generating section 7 at a sender side 2 generates a reception request command in which all transfer data quantity to be transferred to a receiver side and a maximum buffer size of a data transmission memory 18 are written and a transmission section 18 sends the command to a command reception section 13 of a receiver side 3. A reception enable discrimination section 14 reads the reception request command to discriminate the propriety of the data reception and generates a transfer reject command when a data reception memory 19 is busy and returns it to a sender side reply command reception section 9. Moreover, when the memory 19 is idle, the communication size is decided and the written transfer enable command is returned to the reception section 9. The transfer enable command is given to a line acquisition section 11, which acquires a line of a communication system, and all data quantity in the memory 18 is transferred by the transfer number calculated by all the data quantity divided by the communication size written in the transfer enable command.