PURPOSE: To improve the reliability of data transmission and reception, by replacing the function of an adaptor operated independently in normal state with that of other effective adaptor at a failure of the adaptor.

CONSTITUTION: When adaptors 12-1, 12-2 are operated normally, a different value is set to the information (adaptor ID) discriminating the adaptors on the network system. In making a flag representing the effectiveness of the adaptor ID effective to each adaptor, a CPU13-1 and the terminal device have two transmission and reception lines to a data link 11-1, allowing to attain efficient data communication. If one adaptor is failed, the flag representing the effectiveness of the adaptor ID is made ineffective and the adaptor is disconnected logically from the data link section 11-1.