An optical fiber sensor includes: an optical fiber; a light source portion; and a light receiving portion. The optical fiber includes: a core that includes a grating that generates a clad mode upon receipt of light; a clad that covers the core; and a fiber jacket that covers the clad, wherein a part of the fiber jacket corresponding to an area where the grating is formed is removed so that the clad is contactable with the fuel. The light source portion includes a light cutting element that emits light, whose wavelength is within a wavelength band of the cladding mode toward the optical fiber. The light receiving portion that detects intensity of the light transmitted through the grating. The optical fiber, the light receiving portion and the light source portion are arranged linearly.