A fuel cell, comprising: a polymer electrolyte membrane having a catalyst layer (14, 14A, 14C), a gas diffusion layer(15, 15A, 15C), and a separator (21, 21 A, 21C), stacked in order on each of both surfaces of the polymer electrolyte membrane, wherein the gas diffusion layer(15, 15A, 15C) provided at least on one surface side of the polymer electrolyte membrane is structured with a conductive carbon sheet, fluid flow passages being formed on a surface of the gas diffusion layer contacting on the separator (21, 21A, 21C), and a roughness of the surface of the gas diffusion layer having the fluid flow passage formed is smaller than a roughness of a surface of the gas diffusion layer (15, 15A, 15C) contacting on the catalyst layer (14, 14A, 14C), wherein the gas diffusion layer (15, 15A, 15C) is formed by disposing a stacked product made up of a conductive carbon sheet and a resin sheet between a pressing member (41, 51, 61) having a projecting portion (41, 51a, 61a)conforming to a shape of the fluid flow passage and a receiving member (42, 52, 62); pressing the stacked product by the pressing member (41, 51, 61) and the receiving member (52, 52, 62), to form the fluid flow passage at a surface of the conductive carbon sheet, thereby also forming a first and a second skin layer (18, 18A, 18C, 18a) of the conductive carbon sheet, each skin layer (18, 18A, 18C, 18a) being a surface layer having higher density than an inner portion of the conductive carbon sheet, wherein the first skin layer (18, 18A, 18C) is provided on the side of the pressing member (41, 51, 61) and the second skin layer is provided on the side of the receiving member (42, 52, 62) and peeling off the resin sheet from the conductive carbon sheet having the fluid flow passage formed, thereby removing the second skin layer and producing the gas diffusion layer (15, 15A, 15C). a polymer electrolyte membrane having a catalyst layer (14, 14A, 14C), a gas diffusion layer(15, 15A, 15C), and a separator (21, 21 A, 21C), stacked in order on each of both surfaces of the polymer electrolyte membrane, wherein the gas diffusion layer(15, 15A, 15C) provided at least on one surface side of the polymer electrolyte membrane is structured with a conductive carbon sheet, fluid flow passages being formed on a surface of the gas diffusion layer contacting on the separator (21, 21A, 21C), and a roughness of the surface of the gas diffusion layer having the fluid flow passage formed is smaller than a roughness of a surface of the gas diffusion layer (15, 15A, 15C) contacting on the catalyst layer (14, 14A, 14C), wherein the gas diffusion layer (15, 15A, 15C) is formed by disposing a stacked product made up of a conductive carbon sheet and a resin sheet between a pressing member (41, 51, 61) having a projecting portion (41, 51a, 61a)conforming to a shape of the fluid flow passage and a receiving member (42, 52, 62); pressing the stacked product by the pressing member (41, 51, 61) and the receiving member (52, 52, 62), to form the fluid flow passage at a surface of the conductive carbon sheet, thereby also forming a first and a second skin layer (18, 18A, 18C, 18a) of the conductive carbon sheet, each skin layer (18, 18A, 18C, 18a) being a surface layer having higher density than an inner portion of the conductive carbon sheet, wherein the first skin layer (18, 18A, 18C) is provided on the side of the pressing member (41, 51, 61) and the second skin layer is provided on the side of the receiving member (42, 52, 62) and peeling off the resin sheet from the conductive carbon sheet having the fluid flow passage formed, thereby removing the second skin layer and producing the gas diffusion layer (15, 15A, 15C).