To provide a method for producing a fumaric acid monoalkyl ester, capable of efficiently producing the white, powdery ester with high purity.

In this method for producing the fumaric acid monoalkyl ester, a maleic acid monoalkyl ester of which the alkyl group has 10 or more carbon atoms is isomerized in water by using a bromide salt catalyst and/or an iodide salt catalyst together with a free-radical generator so that the fumaric acid monoalkyl ester is formed, then water and an organic solvent not compatible with the water are added to the reaction system and temperature in the system is controlled to be not lower than 40°C so that the isomerized reaction product is made to be dissolved in the organic solvent not compatible with the water, further solvent extraction is conducted, and finally an organic solvent phase which is one of two phases separated after the solution extraction is batched out and cooled so that crystals of the ester are made to be deposited therefrom.