To provide a system capable of keeping measurement precision even in a long-distance transmission by removing a Rayleigh scattered light that becomes a dominant noise in the long-distance transfer using an FBG sensor.

An input light from a wide-band light source is transmitted in an outgoing transmission part and inputted to the FBG sensor through a light circulator. The FBG sensor reflects a light varied in frequency according to a load to the input light. This reflected light is inputted to and transmitted in an incoming transmission part through the light circulator, and the wavelength of the reflected light is detected in a wavelength detector. Since the light circulator is set in the vicinity of the FBG sensor as much as possible, the Rayleigh scattered light generated in the outgoing transmission part is never advanced to the incoming transmission path, and only a pure signal light is transmitted. Accordingly, the signal light free from noise can be detected in the wavelength detector.