To measure temperature at high precision with a simple structure by providing an optical fiber inserted with a tip to the tip vicinity in a radiation tube and a measurement section measuring temperature based on the radiation energy from the other end of the optical fiber.

A radiation tube 1 is made of a metal or a ceramic and has an inner diameter larger than the outer diameter of an optical fiber 2, its tip 1a is closed, and the other end 1b is opened. The tip 2a of the optical fiber 2 is inserted into the radiation tube 1 to the vicinity of the tip 1a. The other ends 1b, 2b of the radiation tube 1 and the optical fiber 2 are held by a hold section 3. The radiation energy from the tip 2a of the optical fiber 2 is condensed by the condensing lens 5 of a measurement section 4 provided on the hold section 3, fed to a detecting element 8 via an aperture 6 and a filter 7, and converted into an electric signal. The electric signal is amplified and digitized by a converting means and converted into a temperature signal. Temperature can be stably measured up to 1500°C at a low cost without being affected by the gas generated by the radiation tube 1 and without polluting the condensing lens 5 of the measurement section 4.