Sr4Ti3O10, which is known to have a Ruddlesden-Popper phase as a layered perovskite-type oxide, showed activity leading to the decomposition of pure water into H-2 and O-2 without any co-catalyst, when irradiated with light under 395 nm. When NiOx was loaded onto Sr4Ti3O10, both by the impregnation (I) method and the vapor deposition (VD) method, this photocatalytic activity drastically increased. Nickel acetylacetonate, when used with the VD method, was found to give rise to more efficient photocatalytic activity than that obtained using nickel nitrate with the impregnation method.