PURPOSE: To make it possible to perform highly accurate measurement in a short time and to provide a structure with few faults, by exciting a fluorescent material in a pulsating mode, guiding the fluorescent light by an optical fiber, measuring the life of the fluorescent light, and obtaining temperature from the measured output and the temperature dependence of the life of the fluorescent light.

CONSTITUTION: A light pulse, which has the pulse width equivalent to the life of fluorescent light of a fluorescent material 1, is outputted from an excited light source 6. The fluorescent material 1 is excited through an optical fiber 2. The fluorescent light generated by the excitation returns through the optical fiber 2 in the reverse direction and received by a light detector 9. The output of the light detector 9 is connected to a measuring unit 13 for the life of the fluorescent light. Temperature is obtained based on the temperature dependence of the life of the fluorescent light by a temperature converting circuit 14.