PURPOSE: To attain the switching of transmission lines in matching with various kinds of applications by disposing a changeover selection switch to give the designation of the classification of system switching to a transmission line switching control circuit.

CONSTITUTION: A transmission line switching control circuit 16 receives switching selection information from a changeover selection switch 17. Then a switching signal is outputted to the transmission line switching selector 14 to switch the transmission line by any of methods that either of the transmission line of 0 and 1 system is fixed for the use, or the transmission line of the 0 system is switched automatically to the transmission of the 1 system to use the transmission line of the 1 system when no fault exists in the transmission line of the 1 system being a standby system and when the fault exists in the transmission line of the 0 system being an active system through the reception of a CRC error or out of frame synchronism, or the transmission line is switched automatically when no fault exists in the transmission line of the system being a standby system and when the fault exists in the transmission line of the other system being an active. Thus, the transmission line is switched on multi- purpose.