PURPOSE: To provide a temperature sensor whereto a high-sensitive optical fiber being excellent in reproducibility is employed, by a method wherein, using a temperature detecting part employing an infrared visual converting fluorescent body, a luminous diode and a semiconductor laser are used as an exciting light source.

CONSTITUTION: A fluorescent body 4 converts infrared light into visual fluorescent light, the infrared light being formed such that either of Tm and H0 and Yb and added to a parent substance crystal, such as a fluoride containing an alkali earth metal and a rare-earth element. The fluorescent body 1 is irradiated with ifrared rays from a semiconductor laser 1 via an optical fiber 2 and a rod lens 3 to radiate strong visual light corrsponding to the temperature of the fluorescent body 4. The light, collected again by the rod 3, is transferred to a branching filter 6 through the optical fiber 5, light from the branching filter 6 is separately detected by a visual light detector 7 and an infrared light detector 8, and temperature is found from the output of the detector 7 with a reference to the output of the detector 8.