PURPOSE: To realize temperature measurement of long distance while enhancing the accuracy by connecting a plurality of small systems, each comprising an optical fiber distributed temperature measuring unit, an optical switch, etc., through an optical fiber.

CONSTITUTION: In each small system 1, an optical switch 3 switches optical fibers 2 and an optical demultiplexer 4 divides the signal into one being transmitted from other small system 1 through the cable 2 and a temperature distribution information signal. An optical fiber distribution temperature measuring unit 5 measures the temperature of the cable 2 while a compute 6 controls the measuring unit 5 and indicates the temperature of the entire system. An optical transmitter 7 transmits the information of respective small systems 1 which form a large system through the cable 2. In each small system 1, the temperature distribution of a cable 2 being connected is measured by switching the optical switch 3 and a temperature distribution data is transmitted to an adjacent small system 1 through the cable 2.