PURPOSE: To detect abnormal high temperature of a body with high sensitivity by using an optical fiber made of a material which deforms at high temperature, bringing the abnormal high-temperature body into contact with the optical fiber, and detecting a decrease in its optical transmission characteristics.

CONSTITUTION: Light from a light emitting device 1 is inputted to a photodetecting device 2 through the optical fiber 3 as a sensor. At this time, the body 4 is brought into contact with or put close to the optical fiber 3 to detect abnormal high temperature. For the purpose, the optical fiber 3 is made suitably of a plastic fiber which has low heat-resisting temperature and deforms easily, e.g. polymethyl methacrylate. Then when the body 4 contacting the optical fiber 3 rises in temperature, the core of the optical fiber 3 deforms and decreases in optical transmission characteristics to reduce the quantity of light photodetected by the photodetector 2. Thus, the decrease in the optical transmission characteristics of the optical fiber is detected to detect the abnormal temperature of the body with high sensitivity.