PURPOSE: To recover a useful component suitable as a raw material for preparing malonic aicd, by heat-treating a heavy sybstance obtained as a by-product in the preparation of acrylic ester in an alkali solution as that it is converted into β-hydroxypropionic acid, acrylic acid, etc.

CONSTITUTION: In preparing an acrylic ester from acrylic acid and an alcohol, a heavy substance obtained as a by-product is treated in an alkali aqueous solution in a pH of 12W14, at 100W200°C under pressure to keep normal pressure or liquid phase for 1W10hr. The heavy substance is converted into an alcohol, β-hydroxypropionic acid, acrylic acid, etc. corresponding rspectively to their esters and recovered as a useful component reusable for an inexpensive raw material for preparing malonic acid or a raw material for preparing acrylic acid. An aqueous solution of hydroxide, oxide, carbonate of alkaline(earth) metal, e.g., NaOH, K2CO3, Ca(OH)2, etc. is used as the alkali aqueous solution.