PURPOSE: To improve the reliability of a system, by powering on a photoelectric converter on a master station side from a master station device and photoelectric converters on respective terminal sides from corresponding terminal devices.

CONSTITUTION: The photoelectric converter 3a on the side of the master station device 1 is supplied with electric power from the master station device 1 by a power source connection 32. The photoelectric converters 3b on the sides of the terminal devices 2 are supplied with electric power from the terminal devices 2 by electric connections 36. Consequently, transmission lines extending to the photoelectric converters 3b of the terminal devices 2 are ready for communication as far as the master station device 10 is powered on, and the operation of the total system is not influenced by the power-on state of a terminal device 2.