To obtain a high-purity quaternary ammonium salt industrially with high efficiency and with a high yield.

In the method for producing quaternary ammonium salt, after the quaternization of tertiary amines, the solution of the quaternary ammonium salt is neutralized with an inorganic acid aqueous solution or with an organic acid aqueous solution, thereby, the obtained aqueous solution of the quaternary ammonium salt is condensed so as to reduce the water content to 10-60%, a solvent is added thereto in the next step, and purification is performed with recrystallization. The electrolytic solution for the electric double layer capacitor can be produced by dissolving the quaternary ammonium salt (A) obtained by the production method in a nonaqueous solvent.