PURPOSE: To realize long time measurement by measuring the temperature distribution through an optical fiber temperature distribution sensor using a light source of specified wavelength thereby suppressing the increase of transmission loss under a radiation environment where the dose rate of γ-ray is high.

CONSTITUTION: A light source 1 employs a pulse oscillation Na+ doped YLiF4 semiconductor-pumped solid-state laser having wavelength of 1.047μm and optical output of +50dBm, for example. The light from the light source 1 impilnges on an optical fiber 3 for sensor through an optical demultiplexer 2. The backscattering light is then demultiplexed by the demultiplexer 2 and converted into electrical signals by O/E converters 4a, 4b. The intensity related to the time is then determined by intensity-time measuring circuits 5a, 5b and the temperature distribution of a fiber 3 is determined based on the intensity ratio between Stokes light and anti-Stokes light at a processing section 6. When a high output light source having wavelength bands of 1 to 1.33μm and 1.48μm or above is employed, a wavelength region having high transmission loss is not superposed on the measuring wavelength band and the transmission loss can be suppressed even under a radiation environment of 100R/h or above thus realizing long time measurement.