PURPOSE: To prevent an opposite-side station from misunderstanding that a fault occurs in a this-side station when the this-side station executes a local loop back process by supplying data containing a surplus bit to be transmitted from a transmitting means to a receiving means and monitoring the data transmitting status in a transmitter-receiver.

CONSTITUTION: Low-order ground data supplied to input terminals 21121n are time-division multiplexed by means of a multiplex conversion (MUX) circuit 23. Status information, etc., are added to the high-order group data obtained as a result of the time-division multiplex by means of an information adding circuit 24. The high-order group data are supplied to an output terminal 26 after the data are converted into codes by means of a coding conversion circuit 25 and, at the same time, to a decoding conversion circuit 32 through a local loop back circuit 41. Thereafter, a control circuit 42 compares the low-order group data supplied to the input terminals 211-21n of the MUX system with low-order group data supplied to the output terminals 361-36n of a separating conversion circuit (DMUX) system. Therefore, occurrence of a fault in this transmitter-receiver can be discriminated.