An optical encoder includes a linear power transmission member which moves longitudinally, a reflector which is provided on an outer peripheral surface of the linear power transmission member and which is longitudinally movable integrally with the linear power transmission member, the reflector including two kinds of reflecting portions different in reflectance are longitudinally arranged, and a light source which radiates light. Further, the optical encoder includes a linear light transmitter which guides the light from the light source to the reflector and to which reflection light reflected from the reflector is guided, and a calculation mechanism configured to calculate a longitudinal displacement of the reflector on the basis of an intensity of the reflection light from the reflector which is guided via the linear light transmitter.