This invention provides thiadiazinone derivatives represented by the following formula (I) ##STR1## wherein R.sup.1 represents a hydrogen atom or C.sub.1 -C.sub.5 alkyl, R.sup.2 represents a 5- or 6-membered heterocyclic ring having (a) 1-3 nitrogen atoms, (b) a oxygen atom, (c) one sulfur atom, (d) 1-3 nitrogen atoms and one oxygen atom, or (e) 1-3 nitrogen atoms and one sulfur atom each of which rings may optionally be substituted by at least one substituent selected from the group consisting of C.sub.1 -C.sub.5 alkyl, cyano, hydroxy, C.sub.1 -C.sub.5 alkoxy, amino, C.sub.1 -C.sub.5 alkylamino, C.sub.2 -C.sub.6 dialkylamino, C.sub.2 -C.sub.5 acylamino, carboxyl, C.sub.2 -C.sub.5 alkoxycarbonyl and carbamoyl; or a pharmaceutically acceptable salt thereof. The compounds according to the present invention have an excellent cardiotonic activity, and are useful as active ingredients of a cardiotonic drug.