A titanium oxide layer containing crystalline anatase was formed on a titanium substrate under hydrothermal conditions. The titanium substrate was reacted in alkaline solutions, potassium hydroxide and a mixture of potassium hydroxide and calcium chloride, at over 120 degrees C. Potassium titanate and calcium titanate were formed on the substrate and they showed the typical texture of each titanate, the aggregate of the fibrous and the cubic particles, respectively. The layers of calcium titanate and the potassium titanate changed into a titanium oxide layer in a solution of the hydrochloric acid at 180 degrees C. Both the potassium and the calcium dissolved from the titanate and then the formed amorphous titanium oxide finally crystallized. The fibrous structure of the potassium titanate did not remain after the acid treatment, although the cubic microstructure of calcium titanate remained. Both samples were mainly composed of fine anatase and the grain size and the crystallite size were almost 40-50 nm and 10 nm, respectively. The photocatalytic degradation of gaseous acetaldehyde by the titanium oxide layer was observed under irradiation of UV light.