An electrolyte composition that shows low methanol cross-over and exhibits high proton conductivity when used as a solid electrolyte for solid polymer fuel cells or the like, and a solid electrolyte membrane and a solid polymer fuel cell that use the electrolyte composition are provided. This electrolyte composition comprises a perfluorocyclobutane-containing polymer having a specific structure. High proton conductivity is provided by sulfonic acid groups connected to the benzene rings. Reduction of methanol crossover is realized by introduction of a rigid structure with aromatic rings, or a combination o a rigid structure with aromatic rings and a three-dimensional cross-linked structure.