A mitotic kinesin Eg5 inhibitor which comprises a thiadiazoline derivative represented by the general formula (I) or a pharmacologically acceptable salt thereof as an active ingredient: [wherein R 1 represents a hydrogen atom and the like, R 2 represents a hydrogen atom, -C(=W)R 6 (wherein W represents an oxygen atom or a sulfur atom, and R 6 represents substituted or unsubstituted lower alkyl and the like) and the like, R 3 represents -C(=Z)R 19 (wherein Z represents an oxygen atom or a sulfur atom, and R 19 represents substituted or unsubstituted lower alkyl and the like) and the like, R 4 represents substituted or unsubstituted lower alkyl and the like, and R 5 represents substituted or unsubstituted aryl and the like] and the like are provided.
