PROBLEM TO BE SOLVED : To provide pharmaceuticals for treating hepatitis C virus, pharmaceutically acceptable salts thereof, and pharmaceutically acceptable prodrugs thereof.SOLUTION : The invention provides a 1' , 2' or 3' -modified nucleoside compound represented by formula I, or pharmaceutically acceptable salts thereof, and pharmaceutically acceptable prodrugs thereof. (R1 to R3 are each independently H, phosphate, acyl, or the like; X1 and X2 are each independently H, linear/branched/cyclic alkyl, or the like; and Y is H, halogen, or the like.)SELECTED DRAWING : NoneCOPYRIGHT : (C)2019, JPO&INPIT