TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Properties: The substrate has thickness of 0.1-5 mm. The primary conductor layer has thickness of 0.01-1 mm. The waterproof film has thickness of 0.1-100 mu m. Preferred Layers: The optical semiconductor electrode further comprises a secondary conductor layer and a secondary semiconductor photocatalyst layer. The substrate is sandwiched between the primary conductor layer and the secondary conductor layer. The secondary conductor layer is interposed between the substrate and the secondary semiconductor photocatalyst layer. The secondary conductor layer and the primary conductor layer are symmetrical with respect to a reference plane. The secondary semiconductor photocatalyst layer and the primary semiconductor photocatalyst layer are symmetrical with respect to the reference plane. The reference plane is located at the center of the thickness direction of the substrate, and is parallel to the surface of the substrate. Preferred Composition: The primary conductor layer comprises element chosen from aluminum, titanium, vanadium, zirconium, niobium and tantalum. At least one portion of the surface which is not in contact with the primary conductor layer or total surface of the substrate is coated with a waterproof film formed using material of primary conductor layer. The primary semiconductor photocatalyst layer is formed using a compound chosen from oxide, nitride and oxynitride, and containing element chosen from titanium, niobium and tantalum. The secondary semiconductor photocatalyst layer is formed using material of primary semiconductor photocatalyst layer. The secondary conductor layer is formed using material of primary conductor layer. The optical semiconductor electrode comprises columnar protrusions on its surface. The surface of each columnar protrusion is formed from the primary optical semiconductor photocatalyst layer. Inside each columnar protrusion, substrates and a portion of primary conductor layer are contained. The substrates contained inside the columnar protrusion are pillar-shaped, and coated with the portion of primary conductor layer contained inside the columnar protrusion. The portion of the primary conductor layer contained inside the columnar protrusion is coated with the primary semiconductor photocatalyst layer formed on the surface of the columnar protrusion.