A syngas was generated at a near-unity H2/CO ratio independent of the reactant gas composition over Rh-loaded strontium titanate in the photo-induced dry reforming of methane. Moreover, light irradiation inhibited side reactions, such as the reverse water gas shift reaction and the precipitation of the solid-state carbon. These results were not explained by thermodynamic equilibrium, but were presumably owing to the unique effect of the photogenerated electron-hole pairs.