Disclosed are a compound and a pharmaceutical composition that exhibit an excellent drug efficacy against a tumor, in particular a tumor which has acquired resistance to gemcitabine. Specifically, provided is a thionucleoside derivative represented by General Formula [1] (in the formula, R 1 represents a hydroxyl group which may be protected, a C 1-20 alkoxy group which may be substituted, or the like； R 2 represents a C 1-20 alkoxy group which may be substituted, a C 3-8 cycloalkoxy group which may be substituted, or the like； and R 3 represents a hydrogen atom or the like)； or a salt thereof. Further, provided is a pharmaceutical composition containing such a thionucleoside derivative or a salt thereof.本發明的課題是提供相對於腫瘤、特別是獲得耐吉西他濱性的腫瘤顯示優異的藥效的化合物及醫藥組合物。本發明提供通式[1]所示的硫代核苷衍生物或其鹽。此外，提供含有這些硫代核苷衍生物或其鹽的醫藥組合物。 [化學式1]（式中，R1表示可以被保護的羥基或可以被取代的C1-20烷氧基等；R2表示可以被取代的C1-20烷氧基或可以被取代的C3-8環烷氧基等；R3表示氫原子等。）
