PURPOSE: To establish the unit which can detect temperature safely at the location where water-proof and explosion-proof are required, by providing gap at the intermediate position of optical fiber and shutting the light flowing to the gap with the plate moved due to temperature change.

CONSTITUTION: The light emitting section 2 is provided at one end of the optical fibers 1, 1' and the photo detector 3 is at another end. There is a gap 4 at the intermediate position of the fibers 1 and 1' and the shut plate 5 provided with bimetal 6 is inserted. When the light from the light emitting section 2 is projected to the cable, if the shut plate 5 is absent in the gap 4, the light through respective optical fiber is transmitted to the photo detector 3. The bimetal 6 is bent depending on the temperature and a part of optical path is shut with the shut plate 5, then the part transmitted with light detects the temperature and the detection signal can perform temperature control. With this constitution, even at the location where water is handled or explosion proof is required, temperature detection can be made with safety.