A transmission device capable of detecting and discarding duplicate data transmitted over Work and Protection paths of a ring network. An add/drop multiplexer adds/drops and cross-connects signals. When an FS-R control signal, for example, is received, a bridge distributes the signal output from the add/drop multiplexer to the Work and Protection routes of the ring network. When the signal distribution is executed by the bridge, a discard information inserter inserts discard information into the signal output to the Work route by the bridge. A discard information detector determines whether or not the discard information is included in the signal dropped by the add/drop multiplexer. A signal discarder discards the dropped signal, depending on whether the discard information is detected by the discard information detector.