NOVELTY - A composite anode is covered on layer of porous semiconductor film on electroconductive electrode base ground layer, and immersed in aqueous solution or aqueous suspension containing biomass, biomass waste material, and organic compound and/or inorganic compound as a fuel. A counter cathode is placed in liquid phase containing aqueous solution or aqueous suspension or liquid-gas phase interface, oxygen is supplied to the liquid phase or gas phase in which the cathode is placed, and fuel cell reaction is carried out. Thus, decomposition and purification of fuel is performed.    USE - Decomposition and purification of fuel used in fuel cell for manufacture of pure metal and micro fuel cell for production of hydrogen (all claimed).    ADVANTAGE - The method efficiently enables decomposition and purification of fuel, without requiring external energy e.g. irradiation with light and with reduced energy consumption.    DETAILED DESCRIPTION - A composite anode is covered on layer of porous semiconductor film on electroconductive electrode base ground layer, and the resultant composite anode is immersed in aqueous solution or aqueous suspension each containing biomass, biomass waste material, and organic compound and/or inorganic compound, as a fuel. A counter cathode for oxygen reduction is placed in liquid phase containing aqueous solution or aqueous suspension or liquid-gas phase interface, oxygen is supplied to the liquid phase or gas phase in which the cathode is placed or oxygen is allowed to coexists in liquid phase or gas phase in which cathode is placed, and fuel cell reaction on the cathode is carried out. Thus, decomposition and purification of fuel is performed. The composite anode contains three layers of electrode base layer, porous semiconductor layer and catalyst layer which forms layer of catalyst film containing metal, metal oxide or semiconductor on the semiconductor layer. INDEPENDENT CLAIMS are included for the following:    (1) fuel cell;    (2) manufacture of pure metal;    (3) production of hydrogen; and    (4) micro fuel cell for production of hydrogen.