PURPOSE: To prevent the issuance of an unnecessary transmission line alarm from an in-station opposite equipment by enabling both terminal stations to independently execute the switching of in-equipment clocks and frame pulses which do not require communication with the opposite stations by only using logic circuits, namely, hardware.

CONSTITUTION: A 1st selector 5 which selects data signals and a 2nd selector 10 which selects clocks and frame pulses given in parallel with the data signals are provided. The 1st selector 5 is provided with an input terminal for inputting a switching command from a switching controlling section 9 and the section 9 is provided with a detecting means which generates the switching command when the means detects the discontinued states of the clocks and frame pulses. The 2nd selector 10 is provided with an input terminal for inputting the switching command from the detecting means. Then both terminal stations execute independently switching by using hardware only. Therefore, a notice alarm synchronized with an in-station clock can be sent to a downstream side and the issuance of an unnecessary alarm can be prevented.