Disclosed is an electrolyte membrane which enables a fuel cell to have a high maximum output when used therein since it has high proton conductivity and high hydrogen gas impermeability. Also disclosed are a method for producing such an electrolyte membrane, and a solid polymer fuel cell using such an electrolyte membrane. A method for producing an electrolyte membrane including a step for impregnating a porous base with a solution containing a sulfonic acid group-containing vinyl monomer and then polymerizing the monomer is characterized in that 80% by mole or more of vinyl sulfonic acid having purity of 90% or more, and/or a salt thereof is contained as the sulfonic acid group-containing vinyl monomer, and the concentration of the vinyl sulfonic acid and/or a salt thereof in the solution is set at 35% by weight or more.