An optical sensor includes a light source, a characteristic light-guiding member, a characteristic changing part which changes the optical characteristic of light, and a detecting unit which detects the light having the optical characteristic changed by the characteristic changing part and guided by the characteristic light-guiding member. The optical sensor includes a control member which inhibits at least the twisting of the characteristic light-guiding member, and controls a bending state of the characteristic light-guiding member, and a positioning mechanism which positions the characteristic changing part with respect to at least a circumferential direction of the characteristic light-guiding member.