An optical fiber sensor that can detect a liquid level or detect a property of liquid with high reliability even under a frequently vibrational environment. The optical fiber sensor includes an optical fiber that includes a core having an area where a grating is formed and a clad, the optical fiber being disposed so that at least a part of the area with the grating is immersed in liquid, a light source for making light incident to the optical fiber so that a cladding mode light having a wavelength band is generated by the grating, and a light receiving unit for detecting the intensity of light which is incident from the light source to the optical fiber and transmitted through the grating.