A solid electrolyte membrane for solid polymer fuel cells, or the like is provided that is chemically stable in a strong acid atmosphere, and has low methanol cross-over as well as high proton conductivity. The solid electrolyte membrane is manufactured, using an electrolyte composition comprising a sulfonic acid group-containing polymer having a specific triazine structure. This polymer can be synthesized, for example, from a sulfonic acid group-containing dihydroxy compound having a triazine structure, and a difluoride.