Photocatalysis of a hollandite compound KxGaxSn8-xO16 (x = ca. 1.8) was examined for the reduction of nitrate ion with a reducing agent of methanol in water under UV irradiation. Hollandites have a characteristic one-dimensional tunnel structure. The hollandite powder, which was prepared by the sol-gel method and unloaded with any additives like metals, was used as the photocatalyst and its photocatalytic reaction was analyzed quantitatively by using ion chromatography and on-line mass spectrometry, and its reaction mechanism was analyzed by in-situ FT-IR, The hollandite photocatalyst showed a significant activity for the formation of N-2 from NO3 -. The nitrate was reduced to N-2 and NO2-, while the reducing agent methanol was partly oxidized to change to formic acid. The conversion of NO3- was proportional to the yields of N-2, NO2-, and HCOO-. The present photocatalyzed decomposition of NO3- -to N-2 would be a useful photocaralysis for the environmental protection of water.