The present invention provides a method of enhancing an antitumor effect by a compound strongly inhibiting ribonucleotide reductase (RNR) or a salt thereof.A combination formulation involving combined administration of a sulfonamide compound represented by Formula (I) [In the formula, X1 represents an oxygen atom or the like； X2 represents an oxygen atom； X3 represents -NH-； X4 represents a hydrogen atom or the like； R1 represents -C(R11) (R12)- or the like； R11 and R12 are the same or different and each represents a hydrogen atom or the like； R2 represents an optionally substituted C6-C14 aromatic hydrocarbon group or the like； R3 represents an optionally substituted C6-C14 aromatic hydrocarbon group or the like； R4 represents a hydrogen atom or the like] or a salt thereof, having RNR inhibitory activity, and other antitumor agent(s).
