Disclosed are a solid electrolyte membrane for fuel cells, which is characterized by that there has been used a silicone resin obtained by subjecting a methide series siloxane compound having a specific, strong acid bis(perfluoroalkanesulfonyl)methide moiety, a specific polysiloxane compound, and a specific silane compound to a cross-linking reaction, and its production process. This membrane has heat resistance, is superior in chemical stability, has a good proton conductivity even under a low water content condition, and has a low methanol permeability.