Abstract: The present invention provides substances having an efficacy against tumors and viruses on which the conventional anti-tumor agents and anti-virus agents exhibit only insufficient effects, and having carcinostatic effect and anti-virus effect on various resistant tumors as well as a reduced side effect such that normal human cells will not be impaired. The present invention relates to an anti-tumor or anti-viral substance represented by the formula (1) wherein R1 represents a nucleic acid base represented by a specific formula, and R2 represents a hydrogen atom, a hydroxy group or a methoxy group, or a pharmaceutically acceptable salt thereof.