A code converter circuit in a digital transmission system in which two mutually-synchronized digital signals are transmitted through a pair of transmission paths is disclosed. The transmission paths are characterized by the possibility of the terminal-to-terminal correspondence between input and output sides of the transmission paths being reversed. The code converter circuit includes an encoder on the input side of the transmission paths and a decoder on the output side of the transmission paths. The encoder comprises a modulo-2 adder and a switching circuit for selectively switching the input digital signals or the adder output signals to the transmission paths. The decoder comprises a modulo-2 subtractor and a switching circuit for selectively switching the digital signals from the transmission paths or the subtractor output signals to reproduce the transmitted digital signals.
