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.
