NOVELTY - An optical semiconductor electrode (200) has electroconductive substrate (102), semiconductor photocatalyst layer (202) formed on surface of the electroconductive substrate, semiconductor photocatalyst layer (203) provided on the surface of semiconductor layer (202), and columnar protrusions. The surface of each columnar protrusion is formed from semiconductor photocatalyst layer (203). The energy difference between Fermi level and vacuum level of electroconductive substrate is smaller than the energy difference between Fermi level and vacuum level of semiconductor photocatalyst layer (202).    USE - Optical semiconductor electrode is used for photoelectrochemical cell used in photolysis of water to generate hydrogen (all claimed) used for hydrogen.    ADVANTAGE - The optical semiconductor electrode has high quantum efficiency and provides photoelectrochemical cell with excellent hydrogen-generation efficiency.    DETAILED DESCRIPTION - An optical semiconductor electrode has electroconductive substrate, semiconductor photocatalyst layer (202) formed on the surface of the electroconductive substrate, semiconductor photocatalyst layer (203) provided on the surface of semiconductor layer (202), and columnar protrusions. The surface of each columnar protrusion is formed from semiconductor photocatalyst layer (203). The energy difference between the Fermi level and vacuum level of the electroconductive substrate is smaller than the energy difference between the Fermi level and a vacuum level of semiconductor photocatalyst layer (202). The energy difference between the Fermi level and a vacuum level of semiconductor photocatalyst layer (202) is smaller than the energy difference between the Fermi level and vacuum level of semiconductor photocatalyst layer (203). The energy difference between the upper end of the valence band and vacuum level of the semiconductor photocatalyst layer (202) is larger than energy difference between the upper end of the valence band and vacuum level of the semiconductor photocatalyst layer (203). The energy difference between the lower end of the conduction band and vacuum level of the semiconductor photocatalyst layer (202) is larger than the difference between the lower end of the conduction band and vacuum level of the semiconductor photocatalyst layer (203). INDEPENDENT CLAIMS are included for the following:    (1) photolysis of water, which involves using photoelectrochemical cell having optical semiconductor electrode, counter electrode electrically-connected to the conductor, liquid which is contacting the surface of the optical semiconductor electrode, and the surface of the counter electrode, and irradiating light to optical semiconductor electrode. The liquid is electrolyte aqueous solution or water;    (2) photoelectrochemical cell used in photolysis of water; and    (3) method for generating hydrogen.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of optical semiconductor electrode.    Electroconductive substrate (102)    Optical semiconductor electrode (200)    Semiconductor photocatalyst layer (201)    Semiconductor layers (202,203)