PURPOSE: To eliminate the hits of other nondefective channel signals by evading the immediate switching of a stand-by unit and then switching this unit in the timing of a blank part of a channel despite the fault of a switching device.

CONSTITUTION: A switch control circuit 20 set between an input buffer circuit 6 and an output buffer circuit 9 is controlled by the signal supplied from a DMUX 5. That is, the DMUX 5 has a function to detect the fault of a package and then outputs an alarm signal (q) when a fault is confirmed in either channel output. A control package outputs a selection signal to a separate package for the switching to a stand-by unit. When an enable signal is supplied from the buffer 6, a signal synchronous with a blank part of 8μs of a channel signal is supplied to the circuit 10 from the DMUX 5. Then the faulty package is switched to the stand-by unit by the enable signal supplied to an output buffer 8 in the timing of the blank part.