The purpose of the present invention is to provide a cationic lipid that can be used as a nucleic acid delivery carrier, a lipid membrane structure in which the cationic lipid is used, a nucleic-acid-introducing agent in which the cationic lipid is used, and a method for achieving nucleic acid introduction using the nucleic-acid-introducing agent containing the cationic lipid. The lipid membrane structure containing the cationic lipid represented by formula (1) (in the formula, each of the symbols is as defined in the specification) has exceptional stability in the blood and tumor accumulation properties. The nucleic-acid-introducing agent in which this cationic lipid is used makes it possible to achieve high nucleic acid delivery efficiency of a nucleic acid to be delivered into the cytoplasm.
