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 R1 represents a hydrogen atom and the like, R2 represents a hydrogen atom, —C(═W)R6 (wherein W represents an oxygen atom or a sulfur atom, and R6 represents substituted or unsubstituted lower alkyl and the like) and the like, R3 represents —C(═Z)R19 (wherein Z represents an oxygen atom or a sulfur atom, and R19 represents substituted or unsubstituted lower alkyl and the like) and the like, R4 represents substituted or unsubstituted lower alkyl and the like, and R5 represents substituted or unsubstituted aryl and the like] and the like are provided.