A method of decomposing a Michael addition reaction product formed as a by-product in the production step of a(meth)acrylic ester and recovering (meth)acrylic acid, a (meth)acrylic ester and an alcohol, in which the formation of an olefin as a by-product can be suppressed. A method of thermally decomposing a by-product at the time of production of a (meth)acrylic ester using an alcohol bearing a branched chain and having 3 or more carbon atoms, wherein the decomposition reaction is carried out in the absence of a catalyst.