A novel photocatalytic microreactor is developed, which comprises two stacked channels divided by a TiO2/Ti bi-layered plate. Under ultraviolet (UV) light irradiation to the TiO2 channel (oxidation channel), the photo-induced holes cause oxidation in the irradiated channel, and residual electrons are transferred through a Ti plate to the other channel (reduction channel) and used for the reduction. The feasibility of this microreactor was studied by combining the oxidation of citric acid in the oxidation channel and the reducing Fe3+ ions in the reduction channel. Fe3+ was reduced to Fe3+ in the reduction channel. The conversion was drastically enhanced by increasing the pH of the citric acid solution in the oxidation channel.