A photochemical reaction system, comprising: a hydrogen generating cell (2, 802) containing a hydrogen generating type photocatalyst, an aqueous medium including a redox compound (5, 805), and an electrolytic solution; the photochemical reaction system being characterised by: an oxygen generating cell (1, 801) containing an oxygen generating type photocatalyst, an aqueous medium including a redox compound (5, 805), and an electrolytic solution; wherein said hydrogen generating cell (2, 802) and said oxygen generating cell (1, 801) are separated by an ion-exchange membrane; a first electrode contained in said hydrogen generating cell (1, 802); a second electrode contained in said oxygen generating cell (1, 801); a temperature difference application part configured to generate a temperature difference between said hydrogen generating cell (2, 802) and said oxygen generating cell (1, 801)to cause the redox compound (5, 805) permeating the ion-exchange membrane and moving around in the electrolytic solution between the first electrode and the second electrode thereby generating a thermoelectromotive force and continuously generating electric power for eliminating the concentration polarization of said redox compound (5, 805) generated as a result of generating a hydrogen gas from said hydrogen generating cell (2, 802) by irradiating said hydrogen generating cell (2, 802) with light and the concentration polarization of said redox compound (5, 805) generated as a result of generating an oxygen gas from said oxygen generating cell (1, 801) by irradiating said oxygen generating cell (1, 801) with light. a hydrogen generating cell (2, 802) containing a hydrogen generating type photocatalyst, an aqueous medium including a redox compound (5, 805), and an electrolytic solution; the photochemical reaction system being characterised by: an oxygen generating cell (1, 801) containing an oxygen generating type photocatalyst, an aqueous medium including a redox compound (5, 805), and an electrolytic solution; wherein said hydrogen generating cell (2, 802) and said oxygen generating cell (1, 801) are separated by an ion-exchange membrane; a first electrode contained in said hydrogen generating cell (1, 802); a second electrode contained in said oxygen generating cell (1, 801); a temperature difference application part configured to generate a temperature difference between said hydrogen generating cell (2, 802) and said oxygen generating cell (1, 801)to cause the redox compound (5, 805) permeating the ion-exchange membrane and moving around in the electrolytic solution between the first electrode and the second electrode thereby generating a thermoelectromotive force and continuously generating electric power for eliminating the concentration polarization of said redox compound (5, 805) generated as a result of generating a hydrogen gas from said hydrogen generating cell (2, 802) by irradiating said hydrogen generating cell (2, 802) with light and the concentration polarization of said redox compound (5, 805) generated as a result of generating an oxygen gas from said oxygen generating cell (1, 801) by irradiating said oxygen generating cell (1, 801) with light.