PURPOSE: To obtain stabilized detection characteristics having no fluctuation by employing the light reflected on the end face of a measuring side optical fiber as a reference light while sharing the measuring side optical fiber between measuring light and reference light.

CONSTITUTION: A measuring fiber 2 has a cut end 2A where a reflective cut face 2Aa of glass is formed. Upon projection of light 2a, 4-5% thereof is reflected to produce a reference light 3a. 94-95% of the projected light 2a impinges on a moving reflector 5 such as a coach cube prism where 10-15% of the light 2a is reflected, as a measuring light 2b, back to the optical fiber 2. The reference light 3a and the measuring light 2b produce an interference wave 1a in the optical fiber 2. The interference wave 1a is utilized in the detection of the absolute position of a rod 6. Since an optical branch coupler is not employed but only a single optical fiber is employed, stabilized interference wave can be obtained with no fluctuation, resulting in downsizing, simplification of structure, and cost reduction.