NOVELTY - The apparatus (100) has an electrode (10) for oxygen generation provided in space (52) of atmosphere containing oxygen and water vapor, and electrode (20) for hydrogen generation provided in space (54) of atmosphere containing hydrogen and water vapor. A porous structure (30) is provided between electrode for oxygen generation and electrode for hydrogen generation. A supply structure is provided to supply electrolyte aqueous solution (60) to pair of electrodes such that a liquid film of electrolyte aqueous solution is formed in electrode surface.    USE - Apparatus for producing hydrogen using optical water-splitting reaction.    ADVANTAGE - Since the reverse reaction by adsorption of product is inhibited by controlling quantity of electrolyte aqueous solution which arrives at the electrode surface, the generation of bubble in the electrode surface can be suppressed and the photoelectric conversion efficiency can be improved. The temperature rise of the electrode can be suppressed by controlling the quantity of the electrolyte aqueous solution passed through the porous structure. The solar energy which cannot be exploited can be suppressed. The voltage drop by solution resistance of electrode surface and the effective light-receiving area can be optimized by optimizing the aperture ratio of the electroconductive support structure.    DETAILED DESCRIPTION - The lighting units (42,44) introduce light to the electrode surface having photocatalyst. The supply structure immerses porous structure in the electrolyte aqueous solution at the time of driving of the apparatus. The electrode for oxygen generation and the electrode for hydrogen generation include electroconductive support structures (14,24) having a penetrated pore such that a catalyst is deposited on the electroconductive support structure having shape of punching metal, mesh shape, lattice shape, shape of expanded metal, fibrous form and spongy. The electroconductive support structure is made of elemental metal and alloys to form elemental metal thin film on the surface of the alloy. The thickness of porous structure is 0.01-10 mm. The surface temperature of electrode for oxygen generation and electrode for hydrogen generation is adjusted by supplying electrolyte aqueous solution in the range of 20-95 degree C. The supply rate of electrolyte aqueous solution is below 2.0asterisk10-3 mol/secasteriskW per energy density of sunlight and more than 4.22asterisk10-8 mol/secasteriskW. The thickness of liquid film of electrolyte aqueous solution of electrode surface is 0.1-500 micrometer.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the hydrogen production apparatus.    Electrode for oxygen generation (10)    Electroconductive support structures (14,24)    Electrode for hydrogen generation (20)    Porous structure (30)    Lighting units (42,44)    Spaces (52,54)