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 .