The present invention provides an electrolyte membrane capable of inhibiting permeation of water, methanol or other electrolyte solutions, permeation of hydrogen and oxygen gas, and swelling caused by electrolyte solution, and having superior mechanical strength, a production process thereof and a membrane-electrode assembly and fuel cell using that electrolyte membrane. The electrolyte membrane has a porous base material having a plurality of pores, and a proton-conducting polymer composition retained in said pores, wherein the proton-conducting polymer composition contains an aromatic hydrocarbon resin having protonic acid groups, free water contained in the electrolyte membrane at 25.degree. C. is present at 0.5 molecules or less per each of the protonic acid groups, bound water contained in the electrolyte membrane at 25.degree. C. is present at 1 molecule or less for each of the protonic acid groups, proton conductivity of the electrolyte membrane in water at 25.degree. C. is 0.001 S/cm or more, and methanol permeability of the electrolyte membrane in 30% by weight methanol at 25.degree. C. is 50 (kg.mu.m/m.sup.2h) or less.