NOVELTY - The method comprises producing a unit cell (10) comprising an anode (15) including an anode catalyst layer (12) containing an anode catalyst, a cathode (19) including a cathode catalyst layer (16) containing a cathode catalyst and an electrolyte membrane (11) interposed between the anode and the cathode. The production of the unit cell comprises immersing the anode catalyst in an acid-containing solution under the presence of a proton-conductive ion-exchange resin. The acid-containing solution has a proton concentration of 0.1-2 mol/L.    USE - The method is useful for fabricating a fuel cell such as solid polymer electrolyte fuel cell and direct methanol fuel cell, which is useful as an in-car power source and a household co-generation system power source for portable electronic devices such as laptop computers, cell phones, personal digital assistant, video cameras and electric scooters.    ADVANTAGE - The method effectively and economically produces energy-efficient fuel cells with lightweight, high output density, excellent power generation performance and long lifetime, and suppresses the performance degradation of the cells.    DETAILED DESCRIPTION - The method comprises producing a unit cell (10) comprising an anode (15) including an anode catalyst layer (12) containing an anode catalyst, a cathode (19) including a cathode catalyst layer (16) containing a cathode catalyst, and an electrolyte membrane (11) interposed between the anode and the cathode. The production of the unit cell comprises immersing the anode catalyst in an acid-containing solution under the presence of a proton-conductive ion-exchange resin. The acid-containing solution has a proton concentration of 0.1-2 mol/L. The immersion of the anode catalyst comprises mixing the anode catalyst and the proton-conductive ion-exchange resin with the acid-containing solution, filtering and removing solids from the mixture of anode catalyst and the proton-conductive ion-exchange resin, preparing a catalyst ink comprising the anode catalyst and the proton-conductive ion-exchange resin, and producing the anode catalyst layer using the catalyst ink. The method further comprises removing anions originating from an acid and the filtered and removed solids, removing anions originating from the acid and from the anode catalyst layer after immersion. The removal of the anions includes a water washing step. The leaching amount of ruthenium from the anode catalyst per hour is less than 1 mu m or per 1 mg of the anode catalyst, when the anode catalyst is immersed in a mixture containing more than 0.1 g of the proton-conductive ion-exchange resin per 1 g of the anode catalyst and protons originating from the acid at a concentration of 0.1-2 mol/L. The acid is an organic acid having not more than 2 carbon atoms. An INDEPENDENT CLAIM is included for a method for fabricating an anode catalyst layer.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a direct methanol fuel cell.    Unit cell (10)    Electrolyte membrane (11)    Anode catalyst layer (12)    Anode (15)    Cathode catalyst layer (16)    Cathode. (19)