PURPOSE: To readily obtain the title high-quality compound in high yield and efficiently by using methanesulfonic acid as a catalyst, reacting maleic acid, etc., with a lower alcohol, neutralizing the reaction product with a solid alkali, dehydrating and then subjecting to vacuum distillation.

CONSTITUTION: Maleic acid, maleic anhydride or fumaric acid is reacted with a lower alcohol such as 1-8C monohydric alcohol, e.g. methanol in the presence of preferably 0.01-1.0wt.% based on the raw material of methanesulfonic acid catalyst preferably at 80-150°C, the reaction product is neutralized with preferably 1.0-10 equivalents based on total acid of the mixture of a solid alkali such as Na2CO3 preferably at 60-120°C, dehydrated under pressure, immediately subjected to vacuum distillation and purified to give the objective compound effective for producing a copolymer useful as adhesive, coating compound, surface treating agent, etc., reducing extremely load of waste water treatment and being excellent in terms of prevention of pollution.