A multiplex communication system comprises a central control unit (2), a plurality of local control units (4-1, ..., 4-n), an optical loop transmission path {8-1, ..., 8-(n+1)} interconnecting the central control unit and the plurality of local control unit by optical fibers in a loop-like configuration to thereby cause data transmission to be effected among the control units through the optical fibers, and an electrical loop transmission path {10-1, ..., 10-(n+ 1)} for interconnecting the central control unit and the plurality of local control units by electrical signal conductors in a loop-like configuration to thereby cause data transmission to be effected among the control units through the electrical signal conductors, wherein the central control unit selects at least one of the optical loop transmission path and the electrical loop transmission path to be connected to the signal transmitter means in dependence on predetermined conditions, so that data transmission is conducted between the central control unit and the local control units through the selected loop transmission path.