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.)
