The present invention is a diamond electrode with high efficiency, a small overvoltage, and a long lifetime, which is reusable, and which can measure the temperature of the electrode. The diamond electrode is at least partially composed of a semiconducting diamond film, whose surface is chemically modified. Another embodiment of the present invention carbon is used as a bare electrode material, diamond crystals are fixed to the bare electrode material, the surface of the undoped diamond crystals are covered with semiconducting diamond film, or semiconducting diamond crystals are fixed to said bare electrode material, and the surfaces of diamond films or crystals are chemically modified. Furthermore, wires may be connected to the diamond electrode to measure the electrical resistance, and hence the temperature.