Optical fiber sensor and brillouin frequency shift measurement method

To uniquely determine a Brillouin frequency shift (BFS) even if a relation between phase and intensity of an intensity signal corresponding to a phase difference between the two optical paths in an interferometer varies. An optical fiber sensor includes a reference section average value calculation unit 180 configured to acquire average intensity in a reference section, a phase control unit 142 configured to control a delay unit in such a manner that a phase difference between two optical paths is swept from 0 to 2π, a Brillouin scattering coefficient elimination unit 176 configured to eliminate a Brillouin scattering coefficient from an interference signal by using an intensity signal, a phase/amplitude calculation unit 184 configured to acquire an initial phase φoffset and amplitude of the interference signal by using a relation between average intensity Iave and the phase obtained through the phase sweep from 0 to 2π, a normalization unit 179 configured to use the amplitude of the interference signal to normalize the interference signal from which the Brillouin scattering coefficient is eliminated, and a BFS computation unit 188 configured to compute a BFS by using the normalized interference signal.