PURPOSE: To enable the monitoring of a long distance and to enhance a reliability, by utilizing optical fiber transmission receiving no effect of electromagnetic induction in a transmission line.

CONSTITUTION: A temp. sensor 3 is provided to a station 2 to be monitored arranged to a remote place and a transmission side optical fiber 5 is laid between the sensor 3 and the light transmitting part 4 arranged to a central monitor station 1. Further, a light receiving side optical fiber 7 is laid between the sensor 3 and the light receiving part 8 arranged to the monitor station 1 and the part where both optical fibers are opposed is brought into close contact with a member 6 deformed by temp. such as a bimetal. When the temp. in the station 2 to be monitored rises abnormally, the member 6 deformed by temp. is bent as shown by a two-dot chain line and the opposed part of both fibers 5, 7 is raised as shown by a two-dot chain line. By this constitution, because a part of the emitted light from the fiber 5 is not incident to the fiber 7, the power of the light received by the receiving part 8 changes. Therefore, by prescribing the loss of light power and temp., temp. can be detected.