##STR00001## (wherein R.sup.1 and R.sup.4 are the same or different and each represents a hydrogen atom, substituted or unsubstituted lower alkyl, substituted or unsubstituted lower alkynyl, substituted or unsubstituted lower alkenyl, or the like; R.sup.5 represents a substituted or unsubstituted heterocyclic group, substituted or unsubstituted aryl, or the like; R.sup.2 represents --C(.dbd.W)R.sup.6 or the like; R.sup.3 represents a hydrogen atom, --C(.dbd.W.sup.A)R.sup.6A, or the like)Antitumor agents which comprises a thiadiazoline derivative represented by the aforementioned general formula (I) or a pharmacologically acceptable salt thereof as an active ingredient are provided.