A polar solvent solution of the present invention is a polar solvent solution in which a solute containing a polyamino acid is dissolved in a polar solvent. The solution has a moisture content of less than 5 mass % based on 100 mass % of the solution. A method for producing a polar solvent solution of the present invention includes changing a moisture content of the solution to adjust the viscosity of the solution. Further, another method for producing a polar solvent solution includes reducing a moisture content of the solution to increase the viscosity of the solution. Thus, the present invention provides a polar solvent solution that enables stable spinning and casting without lowering its viscosity when used as dopes for spinning, film, etc., and methods for producing the same.