To produce malonic acid and an alkylmalonic acid in high yields without generating salts by bringing an aqueous mixture of an ester into contact with a specific acidic ion-exchanger, distilling away the alcohol produced at the time, removing water from the product using an organic solvent and then crystallizing.

The compound is obtained by bringing the aqueous mixture of the ester of formula [R1=H, CH3; R2=H, CH3 or R1+R2=-CH2CH2-; R3=CH3, C2H5, C3H7 or C(CH3)3] (preferably, methyl ester), into contact with the ion-exchanger having a sulfonic acid group at 30-100°C, under 40-1000millbar, (preferably at 50-70°C, under 100-100milliber) the alcohol which is produced at the time, is distilled away, water is removed from the product malonic acid and the alkylmalonic acid by using the organic solvent and crystallizing the product.