Thiadiazinone derivatives

This invention provides thiadiazinone derivatives represented by the following formula (I) wherein R represents hydrogen atom or C1-C5 alkyl, R represents 5- or 6-membered heterocyclic ring having 1-3 nitrogen atoms, a oxygen atom, one sulfur atom, 1-3 nitrogen atoms and one oxygen atom, or 1-3 nitrogen atoms and one sulfur atom and each ring of which may optionally be substituted by at least one substituent selected from the group consisting of C1-C5 alkyl, cyano, hydroxy, C1-C5 alkoxy, amino, C1-C5 alkylamino, C2-C6 dialkylamino, C2-C5 acylamino, carboxyl, C2-C5 alkoxycarbonyl and carbamoyl; or pharmaceutically acceptable salts thereof. The compounds according to the present invention have an excellent cardiotonic activity, and are usful as active ingredients of cardiotonic drug.