PURPOSE: To obtain high transmission quality and also to attain long distance transmission by utilizing light for communication between terminal devices and repeaters, between repeaters and a data transfer device and between the data transfer device and a controller.

CONSTITUTION: When the controller 1 gives a data fetch command, the command is transmitted to a terminal device 7 via the data transfer device 3, an optical fiber cable 4, a repeater 5 and an optical fiber cable 6. The terminal device 7 discriminates a polling code in the command and when a corresponding code exists, the device 7 fetches an output of a measuring instrument 8 and stores it in a latch circuit. Then the controller 1 gives a data transfer request command, the terminal device 7 transfers the stored data to the controller 1 to store it in a recorder 2.