To provide a resin solution of an N-substituted maleimide-olefin copolymer having excellent storage stability and suitable for forming a film having excellent transparency and strength.

The invention provides a resin solution composed of an N-substituted maleimide-olefin copolymer composed of 40-35 mol% olefin residue unit having a specific structure and 60-65 mol% N-substituted maleimide residue unit having a specific structure and an organic solvent and having a polymer concentration of 15-30 wt.% and provides a polymer solution obtained by charging the monomers at an olefin/N-substituted maleimide molar ratio of 0.75-2 and carrying out radical polymerization reaction.