To obtain a pharmaceutical composition useful for treatment of viral infection, especially treatment of hepatitis B by using an oxathiolane nucleoside analog.

This pharmaceutical composition for treatment of infection of hepatitis B comprises a carrier and a compound of formula I {R1 is H or an acyl; R2 is a group of formula II [R3 is H or a (hetero atom-substituted) 1-6C alkyl; R4 is H, a 1-3C alkyl or the like] or a group of formula III (X and Y are each a halogen, amino or OH; when R1 is H and Z is S, R2 is cytosine or the like and when R2 is cytosine, the compound has trans configuration)} or its salt [e.g. cis and trans-2-benzoyloxymethyl-5-(N'4-acetyl-5'-fluoro-cytosin-1'- yl)-1,3-oxathiolane].