Provided is a method for producing an industrially useful ketomalonic acid compound such as ketomalinic acid diesters, or a hydrate thereof, by a method more favorable from an economic and environmental standpoint and from a safety standpoint. The present invention relates to a method involving reacting a malonic acid compound represented by general formula (1) (in the formula, The each Rs indicate an alkyl group, a cycloalkyl group, etc.) with chlorine dioxide to produce a ketomalonic acid compound represented by the general formula (2) (in the formula, R has the same meaning as above), or a hydrate thereof.