The present invention provides a cationic lipid, which allow nucleic acids to be easily introduced into cells, represented by formula (I) (wherein: R 1 and R 2 are, the same or different, alkenyl, etc, and X 1 and X 2 are hydrogen atoms, or are combined together to form a single bond or alkylene, and X 3 is absent or is alkyl, etc, Y is absent or anion, a and b are, the same or different, 0 to 3, and L 3 is a single bond, etc, R 3 is alkyl, etc, L 1 and L 2 are -0-, -CO-O- or -O-CO-), a composition comprising the cationic lipid and a nucleic acid, and a method for introducing a nucleic acid into a cell by using the composition comprising the cationic lipid and the nucleic acid, and the like.