Photo-activated electrons reduce reactants and photo-generated positive holes oxidize them in photocatalysis. The electrons and the holes, however, mostly or partly recombine yielding thermal energy before being used for the reactions because photocatalyst particles are basically symmetry, and thereby cannot generate any effective electric field to separate the electrons and the holes. Both the reduction and the oxidation proceed in the same space simultaneously in conventional photocatalysis reactors using suspended or supported catalyst particles, and thereby reduced products tend to be oxidized and oxidized products to be reduced. These reverse reactions and the infertile recombination of electrons and holes lower photon-efficiency of photocatalysis. In an effort for preventing the reverse reactions and the recombination to raise photon-efficiency, the authors proposed and fabricated a reactor equipped with a hetero-sided photocatalyst plate, that is, a conductive metal plate coated with photocatalyst at its front surface and with a reduction catalyst such as Pt at its back surface. The hetero-sided photocatalyst plate can generate the effective electric field to drive the electrons and the holes opposite directions due to its asymmetrical structure and the Schottky potential generated at the interface between the reactant solution and the photocatalyst. The plate divides the reactor volume into two spaces, oxidation and reduction spaces. The reactor equipped with the hetero-sided photocatalyst plate achieved a high photon-efficiency (the external quantum yield) 20%, five times as large as the undivided reactor, when hydrogen was generated from acidic water in the reduction space while formic acid was oxidized in the oxidation space.