To perform accurate temperature distribution measurement by performing accurate temperature conversion.

An optical fiber 6 for measurement is put through a constant temperature bath 7 of variable temperature adjustment disposed outside of an input/output connector 5 to keep constant the temperature of a part of the optical fiber 6 by the temperature bath 7. Temperature inside the bath 7 is detected by a thermometer 8 as a detection means. By an arithmetic unit 3, the relative temperature of temperature distribution is found from the light intensity ratio and returning time of Raman scatter light returning from the optical fiber 6. Further, with a detection result from the thermometer 8 taken as a reference temperature at an arbitrary position of the optical fiber 6, the temperature distribution is converted to find temperature distribution data. When the data are processed by a data processor 9 based on the distribution data, temperature distribution obtained through data processing is displayed on a monitor 10.