NOVELTY - An electrolytic membrane (2) has a central layer (2c) and surface layers (2a,2b) joined to electrodes (5a,5b) by welding. Surface layer has acidic radical and cross-linkable group. Line swelling rate (E1s) of surface layer containing polar solvent before heating at welding temperature is 15% or more along thickness direction. Line swelling rate (E2) of electrolytic membrane containing solvent after heating at welding temperature is 50% or less. Rate (E2) is less than line swelling rate (E1) of electrolyte membrane containing solvent before heating at welding temperature.    USE - Electrolytic membrane is used for manufacturing membrane electrode assembly used for polymer electrolyte fuel cell (both claimed) used as power supply for vehicles and portable devices.    ADVANTAGE - The electrolytic membrane has high joint strength between membrane and electrode, and excellent fuel barrier property. The electrolytic membrane provides fuel cell having high electric power generation performance.    DETAILED DESCRIPTION - An electrolytic membrane (2) has a central layer (2c) and surface layers (2a,2b) joined to electrodes (5a,5b) by welding. The surface layer has an acidic radical or its precursor group, and a cross-linkable group. The line swelling rate (E1s) of surface layer containing polar solvent before heating at welding temperature is 15% or more along thickness direction. The line swelling rate (E2) of electrolytic membrane containing polar solvent after heating at welding temperature is 50% or less along thickness direction. The rate (E2) is less than line swelling rate (E1) of electrolyte membrane containing polar solvent before heating at welding temperature. The surface layer contains cross-linking compound (X) having metal-oxygen bond, preferably water-soluble titanium compound, and polymeric material having acidic radical and/or its precursor.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic sectional view of membrane electrode assembly.    Membrane electrode assembly (1)    Electrolytic membrane (2)    Surface layers (2a,2b)    Central layer (2c)    Electrodes (5a,5b)