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 (in the formula, R1 represents a hydroxyl group which may be protected, a C1-20 alkoxy group which may be substituted, or the like; R2 represents a C1-20 alkoxy group which may be substituted, a C3-8 cycloalkoxy group which may be substituted, or the like; and R3 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.