RNAi provides a novel molecules can be significantly enhanced the antitumor effects of 5-FU based antitumor agents. RNAi molecule having the nucleotide sequence shown in SEQ ID NO: 2. Anti-tumor agent consisting of 5-FU-based anti-tumor agents and the RNAi molecules.5-FU系抗腫瘍剤の抗腫瘍効果を顕著に増強させることができる新規なRNAi分子の提供。配列番号2に示される塩基配列を有するRNAi分子。当該RNAi分子と5-FU系抗腫瘍剤からなる抗腫瘍剤。