It is made possible to amplify signal light of an optical fiber sensor and to perform measurement in a long distance. At least one of a plurality of core wires in a multicore optical fiber is used as a signal-light propagating core wire 133 and the other is used as a reference-light propagating core wire 134. Also, homodyne detection of signal light and reference light reflected by an FBG sensor unit 132 arranged in each measurement point is performed. Thus, signal light is amplified. In order to make a difference between optical path lengths of the signal light and the reference light equal to or smaller than a coherence length, an optical path-length adjustment unit including a piezoelectric element or the like is arranged in an optical path of the reference light.