To provide a method for producing an aromatic organic solvent solution of a methanesulfonic acid alkyl ester having high thermal stability.

An alkyl alcohol is reacted with methanesulfonyl chloride in the presence of a tertiary amine in an aromatic organic solvent, and the resultant aromatic organic solvent solution of the crude methanesulfonic acid alkyl ester is washed with an aqueous solution of an alkali metal carbonate. In the process, the washing is carried out by using 4-6 pts.mass of a 1-3 mass% aqueous solution of the alkali metal carbonate based on 1 pt.mass of the alkyl alcohol.