To provide a large change in light receiving intensity before and after a temperature rises.

A light source 11 and a light receiving element 12 are connected to each other by optical fiber 13 composed of a core 13a and a clad 13b outside of it, and a temperature detecting part 14 to be arranged in the vicinity of a measuring object is formed in the middle of the optical fiber 13. Here, this temperature detecting part 14 is composed of a taper part 16 which is formed in the middle of the optical fiber 13 and at which a fiber diameter gradually becomes small, a thin diameter part 17 formed over a prescribed length through this taper part 16 and a temperature sensing layer 18 formed by applying a temperature sensing color changing material, which can change light absorption- diffusion to an easily changeable color to the light having a specific wave length by a temperature rise, to the outside of the clad 13b of this thin diameter part 17.