Disclosed is are: an anti-tumor agent comprising, as an active ingredient, a pyrazole derivative represented by the formula (I) [wherein R1 represents an aryl which may have a substituent or the like; R2 represents a cyano or the like; R3 represents a hydrogen atom; R4 represents a hydrogen atom, a lower alkanoyl which may have a substituent or the like; R5 represents an aryl which may have a substituent or an aromatic heterocyclic group which may have a substituent; and X represents -NR9- (wherein R9 represents a hydrogen atom, a lower alkyl which may have a substituent or a lower alkoxycarbonyl which may have a substituent) or -S(O)n- (wherein n represents an integer of 0 to 2)] or a prodrug or pharmaceutically acceptable salt thereof; and others.
