An electrolyte membrane-electrode assembly comprising: a polymer electrolyte membrane; a cathode catalyst layer and a cathode gas diffusion layer including a cathode micro porous layer and a cathode gas diffusion layer substrate, arranged in order on one side of the polymer electrolyte membrane; and an anode catalyst layer and an anode gas diffusion layer including an anode micro porous layer and an anode gas diffusion layer substrate, arranged in order on the other side of the polymer electrolyte membrane, each of the anode micro porous layer and the cathode micro porous layer comprising particulate material; characterized in that i) a relative gas diffusion coefficient of water vapor the anode micro porous layer being smaller than a relative gas diffusion coefficient of water vapor the cathode micro porous layer by an amount equal to or greater than 0.05, (ii) the relative gas diffusion coefficient of water vapor of the anode micro porous layer is 0.05∼0.2; and (iii) the cathode micro porous layer comprises granular conductive material whereas the anode micro porous layer comprises flat conductive material in the form of flat conductive particles to differentiate the relative gas diffusion coefficient of water vapor between the anode micro porous layer and the cathode micro porous layer, wherein the relative gas diffusion coefficient is determined as described in the description by a following mathematical expression Dre = Deff / Dbulk where D re is the relative gas diffusion coefficient, D eff is an effective gas diffusion coefficient [m 2 /s] of water vapor in the anode or cathode micro porous layer, and D bulk is a reference gas diffusion coefficient [m 2 /s] of water vapor in a bulk; wherein the effective gas diffusion coefficient D eff of water vapor in the anode or cathode micro porous layer is measured by using an oxygen diffusion coefficient measuring apparatus and the reference gas diffusion coefficient D bulk is set equal to a gas diffusion coefficient of water vapor in the atmosphere. a polymer electrolyte membrane; a cathode catalyst layer and a cathode gas diffusion layer including a cathode micro porous layer and a cathode gas diffusion layer substrate, arranged in order on one side of the polymer electrolyte membrane; and an anode catalyst layer and an anode gas diffusion layer including an anode micro porous layer and an anode gas diffusion layer substrate, arranged in order on the other side of the polymer electrolyte membrane, each of the anode micro porous layer and the cathode micro porous layer comprising particulate material; characterized in that i) a relative gas diffusion coefficient of water vapor the anode micro porous layer being smaller than a relative gas diffusion coefficient of water vapor the cathode micro porous layer by an amount equal to or greater than 0.05, (ii) the relative gas diffusion coefficient of water vapor of the anode micro porous layer is 0.05∼0.2; and (iii) the cathode micro porous layer comprises granular conductive material whereas the anode micro porous layer comprises flat conductive material in the form of flat conductive particles to differentiate the relative gas diffusion coefficient of water vapor between the anode micro porous layer and the cathode micro porous layer, i) a relative gas diffusion coefficient of water vapor the anode micro porous layer being smaller than a relative gas diffusion coefficient of water vapor the cathode micro porous layer by an amount equal to or greater than 0.05, (ii) the relative gas diffusion coefficient of water vapor of the anode micro porous layer is 0.05∼0.2; and (iii) the cathode micro porous layer comprises granular conductive material whereas the anode micro porous layer comprises flat conductive material in the form of flat conductive particles to differentiate the relative gas diffusion coefficient of water vapor between the anode micro porous layer and the cathode micro porous layer, wherein the relative gas diffusion coefficient is determined as described in the description by a following mathematical expression Dre = Deff / Dbulk where D re is the relative gas diffusion coefficient, D eff is an effective gas diffusion coefficient [m 2 /s] of water vapor in the anode or cathode micro porous layer, and D bulk is a reference gas diffusion coefficient [m 2 /s] of water vapor in a bulk; wherein the effective gas diffusion coefficient D eff of water vapor in the anode or cathode micro porous layer is measured by using an oxygen diffusion coefficient measuring apparatus and the reference gas diffusion coefficient D bulk is set equal to a gas diffusion coefficient of water vapor in the atmosphere.