PURPOSE: To eliminate the possibility of external damage, and to improve the performance by embedding an optical fiber into a notch groove provided on a columnar body cushion layer, placing a temperature sensing body at a prescribed interval on said fiber, and also lumping them together and applying sheath.

CONSTITUTION: A notch groove 2' is provided on the upper part of a cushion layer 2 whose section is circular, consisting of, for instance, foam rubber, and an optical fiber 3 is embedded into the groove. Subsequently, a temperature sensing body 4 consisting of plate pieces 4a, 4b and a KKG (shape memory alloy) wire 4c is placed at a prescribed interval on the optical fiber 3. Next, the cushion layer 2, optical fiber 3 and the temperature sensing body 4 are lumped together, to which a sheath 1 consisting of, for instance, plastic is applied. In such a state, when the temperature of a temperature sensing line rises, the KKG wire 4c extends and exert pressure on the optical fiber 3, the optical fiber 3 sinks into the cushion layer 2, the result is subjected to bending locally, and transmission degree loss increases locally, therefore, the position of a hot spot can be located exactly.