PURPOSE: To attain the transmission of the code error information without using an external device which converts the form of data into the one suited for transmission no using a O-order group transmission line of a PCM multiple transmission line or an external transmission line.

CONSTITUTION: An error arithmetic circuit 17 counts the code errors detected by an error detecting circuit and outputs the transmission quality information. A transmission quality information inserting circuit 18 inserts the transmission quality information received from the circuit 17 into an inter-multiframe station data link bit string of a primary group 24 and sends the information to a transmission quality information processor. A transmission quality information inserting circuit 21 inserts the transmission quality information received from the circuit 17 into the preceding bit string and then transmits the information to another station via a transmission line.