PURPOSE: To attain the economical constitution of a circuit by transmitting all digital signals with nondefective one of both signals devices which transmits the digital signals to a highway after allocating those signals to odd and even time slots in case one of both signal devices has a fault.

CONSTITUTION: Signal transmission parts 10A and 10B of digital signal devices 1A and 1B deliver 12A and 12B plural types of signals An and Bn (An=Bn) in a synchronous state. A processor drives switch clock transmission parts 11A and 11B via signal lines 16A and 16B to produce 13A and 13B even time slots at either one of parts 11A and 11B and odd time slots at the other part. The signals An (even) and Bn (odd) are sent alternately to a highway 3 via highway gate signal lines 14A and 14B and supplied to digital channel networks 2A and 2B respectively. If the device 1B, for example, has a fault, this faulty device 1B is cut off from a system with the line 15B set at an H level. Then the signals given from the part 10A are all sent to the highway 3. Thus a redundant circuit can be formed economically.