An optical fiber sensor includes: a central core disposed at a center of an optical fiber; and an outer peripheral core that spirally surrounds the central core. The effective refractive index ne2 of the outer peripheral core is lower than the effective refractive index ne1 of the central core. A ratio between the effective refractive index ne2 and the effective refractive index ne1 matches a ratio between an optical path length of the central core and an optical path length of the outer peripheral core.