PURPOSE: To improve the reaction rate and the selectivity in the preparation of the titled compound by the reaction of a monohalogenoacetic acid alkyl ester with CO and a lower aliphatic alcohol, by adding a monohalogenoacetic acid, etc. to the reaction system prior to the reaction.
CONSTITUTION: A malonic acid dialkyl ester is prepared by the reaction of a monohalogenoacetic acid alkyl ester with CO and a lower aliphatic alcohol in the presence of cobalt carbonyl catalyst and a basic substance (e.g. alkali metal carbonate.) In the above process, a monohalogenoacetic acid (e.g. monochloroacetic acid, monobromoacetic acid, etc. pref. the one containing same halogen atom as the raw material), a malonic acid monoalkyl ester, or their salts, are added to the reaction system before reaction. The malonic acid monoalkyl ester is pref. a lower alkyl ester, such as methyl ester, ethyl ester, etc., and pref. the one containing the same alkyl group as the objective compound.