(wherein R1 and R4 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; R5 represents a substituted or unsubstituted heterocyclic group, substituted or unsubstituted aryl, or the like; R2 represents —C(═W)R6 or the like; R3 represents a hydrogen atom, —C(═WA)R6A, 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.