Novel tetrahydro-1,3,5-thiadiazin-4-ones of the following general formula (I) or their salts which are useful as an insecticidal and acaricidal agent. ##STR1## In the formula, each of R.sup.1 and R.sup.2 represents a halogen atom or a C.sub.1 -C.sub.4 alkyl group; R.sup.3 represents a halogen atom, a C.sub.1 -C.sub.4 alkyl group, a C.sub.3 -C.sub.6 cycloalkyl group, a C.sub.1 -C.sub.4 alkoxy group, an acetyl group, a phenoxy group, a halo-substituted phenoxy group, a benzyl group, a benzyloxy group, a phenylcarbonyl group, a C.sub.1 -C.sub.4 haloalkyloxy group, a C.sub.1 -C.sub.4 haloalkyloxymethyl group, a C.sub.2 -C.sub.4 haloalkenyloxy group, a C.sub.1 -C.sub.4 haloalkylthio group, a C.sub.1 -C.sub.4 haloalkylthiomethyl group, a C.sub.2 -C.sub.4 haloalkenylthio group, a C.sub.1 -C.sub.8 haloalkyl group, a C.sub.2 -C.sub.8 haloalkenyl group, a C.sub.1 -C.sub.8 alkyloxycarbonyl group, a substituted phenoxycarbonyl group, or a substituted pyridyloxy group; m represents 0, 1, 2 or 3; and n represents 0, 1, 2 or 3.These novel thiadiazines are produced by reacting a compound of general formula (II) ##STR2## wherein R.sup.1 and m are as defined above, with a compound of general formula (III) ##STR3## wherein R.sup.2, R.sup.3 and n are as defined above.