A method produces a 3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compound of following formula (2): ##STR00001## wherein R.sup.1 and R.sup.2 are the same as or different from each other and are each hydrogen atom or an organic group inert to the reaction; and R.sup.3 is hydrogen atom or an organic group inert to the reaction, or a salt thereof, by subjecting a beta-ketoamide-N-sulfonic acid of following Formula (1): ##STR00002## wherein R.sup.1, R.sup.2 and R.sup.3 are as defined above; and X is hydrogen atom, or a salt thereof, to cyclization in the presence of an acid anhydride with or without further subjecting the cyclized product to hydrolysis, in which the cyclization is continuously carried out using a continuous flow reactor.