PURPOSE: To make a thick wiring bundle unnecessary and to prevent the erroneous measurement by making a temp. measuring part to the structure capable of giving the deformation according to the temp. variation against an optical fiber by thermo-sensitive member actuating in accordance with the temp. variation.

CONSTITUTION: A part for measuring temp. (temp. measuring part) 1 is arranged on the optical fiber positioned at the place where the temp. is to be detected. The optical fiber A is passed through a part 2 for guiding the optical fiber, entered the part 1, passed through a part 3 for guiding the optical fiber and led to an outer part. In the part 1, the fiber A is interposed between two sheets of bimetal 4. If the part 1 is heated in this case, the bimetals 4 are deformed, and the fiber A deforms corresponding to the deformation of the bimetals 4. In such a constitution, the thick wiring bundle is made unnecessary, and the erroneous measurement can be prevented.