PURPOSE: To attain communication of the high quality even if a power supply line is used by performing the communication between communication devices at a speed of three times the required communication speed and using the remaining time for the control of errors.

CONSTITUTION: In case of setting up a data rink for example, respective codes of a synchronous character S, a self-data terminal device No. F1, an opposite data terminal device No. F2, and a data rink setting request E are sent from a self-data terminal device DT to a CPU1 in a communication device T at a required communication speed and stored in a two-port memory 5. A CPU2 takes out the data from the memory 5 by the indication from the CPU1 and sends the data after adding (c) an error controlling character B to the receiving side at the communication speed of three times the required speed. Subsequently, the receiving side returns each code (d) in order to respond to the transmitting side and the CPU2 receives the returned code (d). If an error is generated in the process of communication, (c) and (d) are repeated.