A process for easily producing a solid polymer electrolyte having a high ionic conductivity, which comprises reacting an acrylic copolymer comprising structural units represented by the following formula (I) and (II) with a compound represented by the general formula (III): Y-R-Y (in the formula (II) and (III), X and Y are a combination of an isocyanate group and a hydroxyl group) to thereby obtain a solid substance.
