A SONET transmission device has a plurality of INF sections accommodating a plurality of lines and mesh wirings for interconnecting a plurality of INF sections in a meshed pattern. Each of a plurality of interface sections includes a TSI section to output the signal selected from among a plurality of signals through the TSI processing of a plurality of signals on the lines accommodated at all the INF sections, a detector section to detect alarm information on the lines accommodated at all the INF sections, and a control section to provide the TSI section with the control data reflecting the protective switching operation achieved by computation of a logic circuit which hypothetically executes the protective switching operation to be performed based on the alarm information as for a plurality of lines accommodated at its own INF section.
