When titanium oxide is doped with nitrogen, the resulting titanium oxide photocatalyst can be driven with visible light, which is a main component of sunlight. However, in a known process, since a high-temperature heat treatment process necessary for nitrogen doping degrades the photocatalytic activity inherent in titanium oxide, it is difficult to produce a titanium oxide photocatalyst that can be driven with a high efficiency under sunlight.A titania/organic substance composite, which includes an organic ligand coordinated to flaky titania and forms a layered structure, is immersed in aqueous ammonia to substitute the organic ligand between layers with a hydroxyl group by a ligand exchange reaction, and at the same time, to introduce ammonium into between layers of the titania having the layered structure. The resulting composite of titania and ammonium is heated at a temperature of 400.degree. C. or higher and in such a temperature range that does not cause rutile transition, whereby nitrogen is doped into the titania by thermal decomposition of the ammonium and, in addition, titania is crystallized to an anatase form.