Research on the water splitting into H₂ and O₂ using two different semiconductor photocatalysts and a redox mediator, mimicking the Z-scheme mechanism of the photosynthesis, is introduced. It was found that the H₂ evolution took place on a Pt-SrTiO₃(Cr-Ta-doped) photocatalyst using an I⁻ electron donor under the visible light irradiation. And the Pt-WO₃ photocatalyst showed an excellent activity of the O₂ evolution using an IO₃⁻ electron acceptor under visible light. Both H₂ and O₂ evolution was observed with a stoichiometric ratio (H₂/O₂ = 2) for more than 250 h under visible light using a mixture of the Pt-WO₃ and the Pt-SrTiO₃(Cr-Ta-doped) powders suspended in NaI aqueous solution. It is the first finding that the stoichiometric water splitting occurred over oxide semiconductor photocatalysts under the visible light irradiation. We proposed a two-step photo-excitation mechanism using a pair of I⁻/IO₃⁻ redox mediator. The quantum efficiency of the stoichiometric water splitting was ca. 0.1% at 420.7 nm.