PURPOSE: To improve reliability of information transmission where electromagnetic induction is intense, by making a decision on the volume of information from a detector by using an optical fiber cable affected by no electric induction as a substitute for an electric wire.

CONSTITUTION: To send analog values or digital values from information detectors 9 and 17 to a control center, light emitters 11 and 19 and modulators 12 and 21 are used to convert them into photo signals, which are transmitted to optical fiber cables 6 and 7 through driver circuits 14 and 22. In this transmission, a signal detected by detector 9 is supplied via cable 6 to receiver circuit 15, which converts in into an electric signal, and it is reconverted by modulator 21 by the output of oscillator 19 into a photo signal, which is sent out through one optical fiber cable 7 together with the signal of detector 17. In this way, optical fiber calbes 6 and 7 which are not affected by electromagnetic induction at all are used and information from detectors 9 and 17 are discriminated by a frequency-division method, etc., for transmission to a control center.