PURPOSE: To make the restriction on a fluorescent material generous and to make it possible to use a long optical fiber, by exciting the fluorescent material in a pulsating mode, determining the life of the fluorescent light from the fluorescent material based on the time waveform of the generated fluorescent light, and measuring temperature from the temperature dependence of the life of the fluorescent light.

CONSTITUTION: A light pulse, which has the pulse width equivalent to the life of the fluorescent light from a fluorescent material 1, is outputted from an excited light source 6. The fluorescent material 1 is excited through a lens 3 and an optical fiber 2. The fluorescent light, which is generated by the excitation, is inputted to a light detector through the fiber 2 and the lens 3. The output of the light detector 9 is inputted to a measuring unit 13 for the life of the fluorescent light. The life of the fluorescent light is determined by a least square method based on the waveform of an attenuation curve 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.