To industrially advantageously obtain the subject compound by oxidizing glycerol, etc., with a chlorine-based oxidizing agent in the presence of a nitroxide radical and an alkali metal halide, etc.

Glycerol and/or glyceric acid in an amount of 1 mol is oxidized with 2-5 mols of a chlorine-based oxidizing agent (e.g. sodium hypochlorite or calcium hypochlorite) in the presence of preferably 0.005-1.0 mol of a nitroxide radical of the formula (R1 to R6 are each an aliphatic or an aromatic group) (e.g. 2,2,6,6-tetramethyl-pipridin-1-oxyl or 2,2,5,5-tetramethyl-pyrrolidon-1- oxyl) and 0.01-0.5 mol of an alkali metal halide (e.g. calcium chloride or calcium bromide) to give oxy malonic acid. The reaction temperature is preferably 0-40°C.