A transmission device which capsules and decapsules data of a frame and/or a packet includes a decapsuling circuit, wherein the decapsuling circuit includes a bit de-stuff processor configured to perform bit de-stuffing processing on an input data; a byte de-stuff processor configured to perform byte de-stuffing processing on duplicated data of the input data; a check processor configured to perform frame check sequence (FCS) processing on output from the bit de-stuff processor and the byte de-stuff processor; and a mode determination circuit configured to determine a stuffing mode of an opposite transmission device, based on a code conversion result from the bit de-stuff processor and the byte de-stuff processor and the stuffing mode in which the check processor detects the input data with a normal FCS.