Provided are: a novel sulfonamide compound having ribonucleotide reductase inhibitory activity or a salt thereof； and a pharmaceutical composition which contains this sulfonamide compound or a salt thereof as an active ingredient. A compound represented by formula (I) or a salt thereof. (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 may be 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； and R4 represents a hydrogen atom or the like).本發明提供一種具有核糖核苷酸還原酶抑制作用之新穎之磺醯胺化合物或其鹽、及含有其作為有效成分之醫藥組合物。 本發明之化合物或其鹽係以式(I)表示。[式中，X1表示氧原子等；X2表示氧原子；X3表示-NH-；X4表示氫原子等；R1表示-C(R11)(R12)-等；R11與R12相同或不同，表示氫原子等；R2表示可經取代之C6-C14芳香族烴基等；R3表示可經取代之C6-C14芳香族烴基等；R4表示氫原子等)
