PURPOSE: To prepare the title compound useful as a raw material of the intermediate of medicines, easily, in high purity, by extracting glyoxal, in high partition coefficient, from an aqueous solution of crude glyoxylic acid containing glyoxal as an impurity with a specific extractant.

CONSTITUTION: An aqueous solution of glyoxylic acid containing unreacted glyoxal, obtained by reacting an aqueous solution of glyoxal with nitric acid in the presence or absence of oxygen or air, is treated with an extractant selected from propionaldehyde, butylaldehyde, and isobutylaldehyde, to obtain the purified title compound. The volume ratio of the extractant to the crude glyoxylic acid solution is pref. 2W3 to 1, and the weight ratio of the extractant to the glyoxal in the crude glyoxylic acid is pref. 40W200 to 1.