The problem to be solved by the present invention is to provide a therapeutic agent and a pharmaceutical composition both for diseases associated with the damaging of DNA, particularly solid cancer. The present invention provides a DNA-damaging agent comprising 1-[2-deoxy-4-thio-1-beta-D-ribofuranosyl]-5-fluorouracil or a salt thereof as an active ingredient.本發明欲解決之課題為提供治療劑、醫藥組成物給肇因於DNA損傷之疾病,特別是固態癌。本發明提供之DNA損傷劑是1-[2’-去氧-4’-硫代-1-β-D-核呋喃糖基]-5-氟尿嘧啶或其鹽為有效成分。