The present invention addresses the problem of providing: a compound and a pharmaceutical composition that exhibit superior efficacy against tumors, especially tumors that have acquired gemcitabine resistance. The present invention provides thionucleoside derivatives represented by general formula (1), or salts thereof. (In the formula: R1 represents an optionally protected hydroxyl group, an optionally substituted C1-20 alkoxy group, or the like； R2 represents an optionally substituted C1-20 alkoxy group, an optionally substituted C3-8 cycloalkoxy group, or the like； and R3 represents a hydrogen atom or the like). Further provided are pharmaceutical compositions containing these thionucleoside derivatives or salts thereof.
