A high quality 3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compound or a salt thereof is obtained easily and efficiently.When the compound represented by the following formula (2)[Formula 2] ##STR00001## (wherein, R.sup.1, R.sup.2, and R.sup.3 are hydrogen atom or an organic group inert to the reaction, and X is hydrogen atom) or a salt thereof is produced by cyclization of a mixture of .beta.-ketoamide-N-sulfonic acid represented by the following formula (1) [Formula 1] ##STR00002## or a salt thereof and an inert solvent and a mixture of acid anhydride and an inert solvent, and by subsequent hydrolysis of the product, a step of (A) hydrolyzing the reaction product obtained by the cyclization by mixing with an aqueous solution of sulfuric acid so as a concentration of sulfuric acid in an aqueous phase after the hydrolysis would become 30% by weight or more, and then separating an organic phase and an aqueous phase, or a step of (B) washing the organic phase liquid after the hydrolysis with an aqueous solution of sulfuric acid with a concentration of 30% by weight or more is at least carried out.