An optical sensor includes an optical fiber disposed in such a way that an end surface thereof is exposed in a distal end surface (10a) of a sensor head (10). The optical sensor includes: a first plate material (first metal plate) (20) in close parallel contact with the distal end surface of the sensor head, and in which a through-hole is formed in a position corresponding to the end surface of the optical fiber; and a second plate material (second metal plate) (30) disposed parallel to the first plate material (first metal plate) and toward the front thereof in a measuring direction, and in which a through-hole is formed at a point of intersection with a virtual straight line perpendicular to the end surface of the optical fiber. With this optical sensor, it is possible for clearance measurement to be performed even if the optical fiber becomes opaque or worn.