The object of present invention is to provide a system that can deliver in vivo nucleic acids such as an siRNA for suppressing a target gene expression in vivo more safely and efficiently, and to provide an expression-suppressing agent and a pharmaceutical composition utilizing the system. An introduction substance into chylomiclon, particularly nucleic acids to which an alpha-tocopherol is bound for suppressing a target gene expression, can be delivered more safely and efficiently into hepatic cells in vivo by administering the nucleic aids under the condition where the production of chylomicron is induced in the body. Alternatively, alpha-tocopherol-bound nucleic acids are mixed with extracted chylomiclon, and then they are administered. Consequently, a target gene expression is suppressed, thereby a disease caused by an elevated expression of the target gene can be treated more safely and efficiently.
