The present invention provides an antitumor agent and the like, which comprises as an active ingredient, a pyrimidine derivative represented by Formula (I): [wherein —X—Y—Z— represents —O—CR3═N— (wherein R3 represents a hydrogen atom, a substituted or unsubstituted aromatic heterocyclic group and the like) and the like, R1 represents —NR10R11 (wherein R10 and R11 may be the same or different, and each represents a hydrogen atom, substituted or unsubstituted lower alkyl and the like) and the like, R2 represents —NR13R14 (wherein R13 and R14 may be the same or different, and each represents a hydrogen atom, substituted or unsubstituted lower alkyl and the like)] or a pharmaceutically acceptable salt thereof.