An objective of the present invention is to provide an optical fiber sensor which has a simple configuration to enable sensitively measuring a refractive index of a measurement medium in a wide range of refractive indexes. An optical fiber sensor according to the present invention includes; an optical fiber having a core in which a short-period gratings are formed and a cladding, the optical fiber being made so that transmission loss occurs due to cladding-propagation-mode leakage at its cladding portion where the short-period gratings are formed; a light source from which light having a wavelength range of the cladding propagation mode is emitted; and a light receiving unit for receiving transmission or reflection light having passed through the cladding at the position where the short-period gratings are formed. The optical fiber sensor detects, based on the total receiving light intensity that the light receiving unit receives, the refractive index of the measurement medium contacting the cladding, in which the optical fiber is multi-mode one, and the short-period gratings have plural kinds of periods.