PURPOSE: To allow other part of system to operate entirely normally even if a slave station is faulty by connecting optical fibers by the 1st optical directional coupler of a slave station, coupling the input/output of a data processing circuit with an optical fiber by 2nd optical directional coupler and allowing a master station to use the optical directional coupler to couple the input/output of the central with the optical fiber.

CONSTITUTION: A 4th terminal among 4 terminals of an optical directional coupler 20 in a slave station 11 is used, the 4th terminal is connected to the input terminal of a photoelectric converter 10 and an electric output of a photoelectric converter 14 is fetched in a data processing circuit 15. As a result, each slave station 11 can receive information from a slave station at a remoter position from its own station with respect to a master station 12. Thus, a data sent from a slave station can receive all slave stations toward the master station from the slave station. Moreover, the data sent from all the slave stations. Furthermore, the data sent from all the slave stations is received by the master station 12 via the 1st terminal 21 of the optical directional coupler 20 of the slave station, the 3rd terminal and an optical fiber 10. Thus, even if a slave station is faulty, since other parts of the system are acted normally, the reliability is improved.