PURPOSE: To make the accurate measurement in strong magnetic fields and the safe and easy measurement in environments accompanying high temp., high pressure and high voltage possible by transmitting the output of a light emitting element of which the spectral distribution state changes with the temp. of an object to be measured by means of an optical fiber.

CONSTITUTION: The light emission characteristics of a light emitting element 2 changes like T1WT4 with temps., and when the temp. T of an object 1 to be measured rises, the peak wavelength λ of the relative light emission intensity of the element 2 moves successively to the long wavelength side. The output of the element 2 is transmitted to a detector 7 through an optical fiber 3, by which the separation of the emitted light into its spectral components and the detection of the wavelength at which the relative light emission intensity becomes max. are accompliesed. As a result, the direct reading of the temp. of th object 1 is made possible by connecting a converter which converts the peak wavelength λ of the relative light emission intensity to the temp. T and displays the temp. value to the detector 7.