[en] Fabrication of a nanoporous tungsten oxide film via anodizing of tungsten in an ammonium nitrate (NH₄NO₃)/ethylene glycol (EG)/water (H₂O) mixture is reported for use as a visible light-driven photocatalyst. Anodizing of tungsten in a 1.0 M NH₄NO₃/EG solution containing less than 0.1 vol% H₂O resulted in active dissolution of the tungsten substrate. As the H₂O concentration increased to more than 25 vol%, a thin barrier oxide film was formed on the tungsten substrate. A thick porous tungsten oxide with numerous nanopores measuring several tens of nanometer in diameter was fabricated via anodizing at a moderate H₂O concentration of 1.0 vol%. The porous oxide consisted of a double-layered structure with an outer porous layer and an inner dense layer, and the outer porous layer became thinner as the NH₄NO₃ concentration decreased. A uniform porous oxide film from the top surface to the bottom interface was fabricated via anodizing at 20 V in a 0.02 M NH₄NO₃/EG solution containing 1.0 vol% H₂O at 313 K. The porous tungsten oxide exhibited visible light-driven photocatalytic activity for the photocatalytic decomposition of methylene blue.