PROBLEM TO BE SOLVED: To provide an electrolyte composition solution production method that can obtain a transparent electrolyte composition solution having excellent temporal stability with high productivity, an electrolyte composition solution obtained by the production method, and an electrolyte membrane and a solid polymer electrolyte fuel cell using the electrolyte composition solution.SOLUTION: An electrolyte composition solution production method includes: a supply step of continuously supplying a solution (A) containing a compound (a) having an acidic functional group and a solution (B) containing a compound (b) having a basic functional group to a mixing unit, respectively; and a mixing step of mixing the solution (A) and the solution (B) in the mixing unit to obtain an electrolyte composition solution. The linear velocity of the mixing unit is not less than 0.020 m/s and not more than 5.0 m/s.
