An object of the present invention is to provide a nucleic acid delivery carrier composition of low toxicity and high safety, the carrier composition, when used to administer a nucleic acid such as an siRNA into an animal-derived cell or organism, being capable of delivering efficiently the nucleic acids into the cells while protecting it from being degraded; and a nucleic acid deliver composition containing the carrier and a nucleic acid. The carrier composition for delivery of a nucleic acid is prepared by mixing (A) a cationic lipid having a steroid skeleton with (B) a tertiary ammonium salt-type cationic lipid. The nucleic acid delivery composition is prepared by mixing the nucleic acid delivery carrier with a nucleic acid.