PURPOSE: To provide a temperature measuring apparatus capable of enduring against severe environment with high sensitivity such as, for example, an effective temperature measuring apparatus regarding an electromagnetic field like the core of a transformer or the applied example of superconduction.

CONSTITUTION: The apparatus for measuring temperature comprises a light emitting unit 30, a nitrogen-containing diamond sensor 20, a light receiving unit 32, and length parts 34, 35 of an optical fiber for coupling the units 30, 32 to the sensor 20. The sensor 20 has a base for passing the emitted light and reflected light, and oblique surfaces 24, 26 guided to a vertex. The light quantity absorbed by the sensor 20 is varied in response to the temperature of the place where the sensor 20 is disposed.