To grasp temperature fluctuation in a cable accurately and easily by providing a temperature sensor comprising an optical fiber for detecting temperature distribution in the longitudinal direction in the cable.

A temperature sensor 2 comprising an optical fiber 2a is spirally or linearly arranged over the longitudinal direction of a highly viscous oil immersion-insulation cable 1. Then, when light output pulse beams from a light source 31 are applied to the temperature sensor 2 in the cable 1 via a light- branching filter 32, Raman scattering light with an intensity suited for the temperature in the cable 1 is generated in the temperature sensor 2 and is partically emitted from the input terminal of the temperature sensor 2 as back scattering beams. The light-branching filter 32 separates the scattering light into Stokes light and anti-Stokes light, converts them into electrical signals by light reception elements 33 and 34, and performs averaging processing, and then transfers them to a computer 39. The computer 39 calculates temperature in the cable 1, the change rate, or the amount of temperature change within a specific amount of time base on the transferred electrical signals.