PURPOSE: To detect the detecting point where abnormality is yielded, by providing attenuators having different amounts of attenuation at detecting points, respectively, in such a way that the attenuator at the detector where the abnormality occurred is operated, and the light level which is detected by an observing part is attenuated by the amount of attenuation of said attenuator.
CONSTITUTION: An optical fiber (f) is laid so that a loop comprising an observing part C, detecting points A1, A2... where attenuators B1, B2... having different amount of attenuation are provided, and the observing part C are formed. Light is transmitted from one end of the optical fiber (f) to the other end. The level of the light is always detected at the observing part C. When abnormality is yielded at any detecting point Ax, the amount of attenuation in the light level is detected at the observing part C.
MATSUBA HIRONORI
KOKAYU MIKIO
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