This method provides a method for delivering shRNA targeting TS in vivo. In addition, the following is provided: an antitumor agent, which comprises short hairpin RNA (shRNA) capable of inhibiting expression of thymidylate synthase by RNAi action and a PEG-modified cationic liposome, wherein the shRNA is bound to the surface of the PEG-modified cationic liposome and has an overhang comprising at least two nucleotides at the 3 end.