PURPOSE: To avoid the useless transcription of the transmission data or the protocol header between buffers by using a buffer at its back side to add the protocol header to the transmission data.

CONSTITUTION: An interface control part 21 is connected to a transmission control part 22 which performs the extraction and addition of transmission control characters, the decision of a transmission sequence and detection of errors by the commands given from a central processing unit 10. The part 22 contains a transmission buffer 23 which adds a protocol header to the transmission data in a data transmission mode under the control of the part 22. The part 22 is connected to a circuit control part 24 and this part 24 is connected to an MODEM (not shown here). for serial/parallel conversion of signals, the connection of circuits and monitor of the communication state respectively. Thus the buffer 23 is used at its back side to add the protocol header, etc., to the transmission data. This can avoid the useless transcription of the transmission data, the protocol header, etc., between buffers.