To expand a measurement range with regard to SDH-BOTDR.
A BFS1 and a BFS2, which is obtained by inverting the BFS1, are acquired on the basis of a first measurement signal and a first local oscillation signal that is a cosine wave, a BFS3 is acquired on the basis of a second measurement signal and a second local oscillation signal that is a sine wave, and a Brillouin frequency shift waveform is synthesized from the BFS1 to BFS3. In the case where an optical fiber is not strained or in the case where temperature of the optical fiber is not changed, a phase rotation number N, which is a phase difference between a measurement signal and a local oscillation signal, is calculated on the basis of intensity of the measurement signal, and an offset corresponding to the phase rotation number N is given to the synthesized Brillouin frequency shift waveform.