To provide a medicine which is effective to animal cancer and viral diseases.

A trifluorothymidine derivative of formula (I) as medical agent. In the formula (I), R1 indicates H, a lower alkyl, a lower alkanoil, etc.; R2 is H, a lower alkyl substituted sylil, a lower alkylphenyl substituted sylil, etc.; R3 is H, acetyl, or benzoyl, etc.