PURPOSE: To measure temperature without obstruction due to electromagnetic induction by a method wherein optical fibre is arranged around a thermal expanding body put or attached to arranged around a thermal expanding body put or attached to equipment to be measured, and light is iradiated from one end so as to measure its attenuating ratio.

CONSTITUTION: A thermal expanding body 3 is arranged in or attached to equipment to be measured. The thermal expanding body 3 consists of metal, plastics etc., and optical fibre 2 is coiled around the body. By this construction, stress is applied to the optical fibre 2 by the thermal expanding body 3 when it reaches a certain high temperature, and the transmission loss of the optical fibre 2 increases. In other words, when light is irradiated to the optical fibre 2, there is not much difference in the attenuating ratio of light within the region of constant normal temperature, but the attenuating ratio suddenly increases at a certain high temperature. Therefore, accurate temperature-measuring is possible without the influence of electromagnetic induction.