PURPOSE: To accurately detect the direction and change velocity of the change in temp. applied to an optical fiber, by observing the continuous moving direction and change velocity of the intensity of return light from a fiber etalon.

CONSTITUTION: The beam from a He-Ne beam source 1 is incident to an optical fiber 12 through a lens 5. A phase modulator 10 is provided to the intermediate part of the optical fiber 12 and the output beam of the optical fiber 12 is incident to a sensor 9 from one terminal thereof through a lens 6, a half mirror 4 and a lens 7. The incident beam is reflected at the other terminal of the sensor 9 to be incident to a detector 11 through the lens 7, the half mirror 4, a total mirror 3 and a lens 8. Herein, the detector 11 is one with a system using a charge coupling element and, therefore, in the detector 11, the interference fringe between both lefthand and righthand skew rays emitted from the incident terminal surface can be observed and detected in an enlarged state. Further, by controlling the total mirror 3, the interference fringe can be projected on a television camera.