Disclosed is a process for the production of ketomalonic acid compounds or hydrates thereof by reacting a malonic acid compound with one or more chlorous acid compounds selected from among chlorous acid and chlorites and thus oxidizing the methylene group of the malonic acid compound. The process does not necessitate highly toxic reagents, lowly safe reagents, special reactants, special reaction equipment, expensive reagents, expensive catalysts, or transition metals such as noble metals, and permits the selection of mild reaction conditions and simple operation, thus enabling efficient and easy production of ketomalonic acid compounds such as ketomalonic diesters under simple and easy conditions suitable for industrialization.