[Problem] To permit reduction in the temperature dependence of a sensor, which is ascribable to the temperature dependence of the Verdet constant of a sensor fiber, at a low cost.[Means for Resolution] In a prior-art optical sensor fiber of reflection type, reduction in the temperature dependence of a sensor is coped with by, e.g., duplexing a signal processing circuit and a receiving optical system, so as to execute the mean processing of modulation signals, whereas in this invention, the elimination of the temperature dependence of a modulation signal is realized by selecting a ferromagnetic Faraday rotor 13 so that a modulation degree (S.sub.out) expressed by the ratio between an AC component and a DC component may become constant or within a certain range, so as to especially simplify a configuration and to attain a lower cost.