A highly active titanium oxide photocatalyst of the type responsive to visible light is prepared by subjecting a titanium (hydr)oxide raw material obtained by neutralizing an acidic titanium compound in ammonia or an amine under conditions such that the final pH is 7 or below to heat treatment in an atmosphere containing a hydrolyzable compound followed by contact with water and additional heat treatment at a temperature of at least 350.degree. C. The resulting titanium oxide photocatalyst comprises titanium oxide with a specific surface area of at most 120 m.sup.2/g and with the amount of surface hydroxyl groups being at least 600 .mu.eq/g. Preferably the density of surface hydroxyl groups is at least 8 .mu.eq/m.sup.2, and the ratio of the amount of terminal type hydroxyl groups (T) to the amount of bridge type hydroxyl groups (B) in the surface hydroxyl groups satisfies T/B.gtoreq.0.20. This titanium oxide photocatalyst has an ESR spectrum having two types of triplet signal for which the g values of the main spectra are 1.993-2.003 and 2.003-2.011, respectively, and it is also identified by having ratios of the strengths of these signals before irradiation with visible light, under irradiation, and immediately after the stop of irradiation which are different from conventional products.