The present invention comprises: a light source； a generator that generates, from light generated by the light source, a light pulse train in which the carrier waves are coherent, interference between adjacent waveforms is low, and the spatial length of a pulse width is smaller than a depth range of an observation target region in a measurement target； a frequency shifter that converts the frequency of a light pulse train modulated by the generator； a light path length changing unit that changes the light path length of the light pulse train； a light detection unit into which is input the light pulse train output from the light path length changing unit and backwardly scattered waves from the measurement target； a filter that extracts a difference signal output from the light detector and having a shift frequency of the frequency shifter； a demodulator that combines the difference signal extracted by the filter and a reference signal synchronized with the shift frequency of the frequency shifter； and an analyzing unit that analyzes a signal output by the demodulator.
