To provide a manufacturing method of a solid polymer electrolyte membrane formed of an acrylamide-based polymer having a mono or diphosphonic acid group or a sulfonic acid group, and having high proton conductivity.

Mono or diphosphonic acid group-containing (meta) acrylamide obtained by dissolving a (meta)acrylamide-based monomer that may be N-substituted in a vinyl sulfonic acid in the absence of a solvent, reacting phosphoric anhydride therewith and subjecting it to hydrolysis is copolymerized with the vinyl sulfonic acid, whereby the solid polymer electrolyte membrane having high proton conductivity is provided.