PURPOSE: To mismatch a path for light propagation and permit a warning to be issued at the time of a fire, by connecting optical fibers by a glass sphere into which a medium with a high coefficient of thermal expansion is sealed.

CONSTITUTION: A liquid with a high coefficient of thermal expansion is closed up tightly in a thermosensitive glass sphere 1, and with expansion of the liquid at a predetermined temperature, the glass sphere is destoryed. The sphere 1 combines optical fibers 3 and 4 that penetrate a box into a body in cooperation with a spring 2. Through fibers 3 and 4, light from a light source 5 is introduced into a light receiving device 6, and the received signal is transmitted to a detector 7. The temperature sensor is set at a desired position, and the sphere 1 is destroyed when fire coccurs followed by disconnection between the fibers 3 and 4, and further transmitted light is interrupted, and a detected output becomes a value under a set value, thereby allowing a warning to be issued.