To obtain a solution of an N-substituted maleimide-olefin copolymer having excellent storage stability in long-term preservation and to provide a method for producing a film using the same.

The solution of an N-substituted maleimide-olefin copolymer comprises 10-40 wt.% of an N-substituted maleimide-olefin copolymer and 90-60 wt.% of a mixed solvent composed of 1-40 wt.% of a nitrile-based solvent and 99-60 wt.% of a halogen-based solvent and has excellent storage stability. The method for producing the film of an N-substituted maleimide-olefin copolymer comprises using the solution of the N-substituted maleimide-olefin copolymer and removing the solvent from the solution.