To obtain an unsaturated carboxylic acid (ester) causing little environmental pollution.

This unsaturated carboxylic acid (ester) is obtained by the three steps composed of amidation step: (meth)acrylonitrile is catalytically hydrated in the presence of a solid catalyst to synthesize (meth)acrylamide; hydrolysis step: the (meth)acrylamide is hydrolyzed in the presence of basic catalyst to synthesize a (meth)acrylic acid salt and ammonia; and electrodialysis step: the (meth)acrylic acid salt is decomposed into (meth)acrylic acid and a base as catalyst by electrodialysis.