A method for preparing a composition for delivering an anionic drug, wherein said method (i) comprises: (a) dissolving an anionic drug and a cationic lipid in an aqueous solvent respectively and mixing them; and (b) dissolving an amphiphilic block copolymer in an aqueous solvent and mixing with the mixture obtained in step (a), wherein the cationic lipid is a cationic lipid represented by Chemical Formula 1: in the formula 1, n, m and 1 are each 0 to 12, with a proviso that 1 ≤ n + m + 1 ≤ 12, a, b and c are independently 1 to 6, R 1, R 2 and R 3 are independently hydrogen or a saturated and unsaturated hydrocarbon having 11 to 25 carbon atoms, with a proviso that at least one of R 1, R 2 and R 3 is a saturated or unsaturated hydrocarbon having 11 to 25 carbon atoms, and wherein the amphiphilic block copolymer is an A-B type block copolymer comprising a hydrophilic block and a hydrophobic block where the hydrophilic A block is polyethylene glycol or monomethoxypolyethyleneglycol, and the hydrophobic B block is polylactide, polygylcolide or coplolymer thereof; and wherein the steps (a) and (b) are conducted in a monophase, or said method (ii) comprises: (a') dissolving an anionic drug and the cationic lipid represented by Chemical Formula 1 as defined above in an aqueous solvent respectively and mixing them, followed by freeze-drying; (b-1) dissolving the freeze-dried product obtained in step (a') in an organic solvent; (b-2) mixing the solution obtained in step (b-1) with an aqueous solvent; and (b-3) removing the organic solvent from the mixture obtained in step (b-2), wherein the amphiphilic block copolymer is dissolved in the organic solvent of step (b-1) or the aqueous solvent of step (b-2). wherein said method (i) comprises: (a) dissolving an anionic drug and a cationic lipid in an aqueous solvent respectively and mixing them; and (b) dissolving an amphiphilic block copolymer in an aqueous solvent and mixing with the mixture obtained in step (a), wherein the cationic lipid is a cationic lipid represented by Chemical Formula 1: in the formula 1, n, m and 1 are each 0 to 12, with a proviso that 1 ≤ n + m + 1 ≤ 12, a, b and c are independently 1 to 6, R 1, R 2 and R 3 are independently hydrogen or a saturated and unsaturated hydrocarbon having 11 to 25 carbon atoms, with a proviso that at least one of R 1, R 2 and R 3 is a saturated or unsaturated hydrocarbon having 11 to 25 carbon atoms, and (a) dissolving an anionic drug and a cationic lipid in an aqueous solvent respectively and mixing them; and (b) dissolving an amphiphilic block copolymer in an aqueous solvent and mixing with the mixture obtained in step (a), wherein the cationic lipid is a cationic lipid represented by Chemical Formula 1: in the formula 1, n, m and 1 are each 0 to 12, with a proviso that 1 ≤ n + m + 1 ≤ 12, a, b and c are independently 1 to 6, R 1, R 2 and R 3 are independently hydrogen or a saturated and unsaturated hydrocarbon having 11 to 25 carbon atoms, with a proviso that at least one of R 1, R 2 and R 3 is a saturated or unsaturated hydrocarbon having 11 to 25 carbon atoms, and n, m and 1 are each 0 to 12, with a proviso that 1 ≤ n + m + 1 ≤ 12, a, b and c are independently 1 to 6, R 1, R 2 and R 3 are independently hydrogen or a saturated and unsaturated hydrocarbon having 11 to 25 carbon atoms, with a proviso that at least one of R 1, R 2 and R 3 is a saturated or unsaturated hydrocarbon having 11 to 25 carbon atoms, and wherein the amphiphilic block copolymer is an A-B type block copolymer comprising a hydrophilic block and a hydrophobic block where the hydrophilic A block is polyethylene glycol or monomethoxypolyethyleneglycol, and the hydrophobic B block is polylactide, polygylcolide or coplolymer thereof; and wherein the steps (a) and (b) are conducted in a monophase, or said method (ii) comprises: (a') dissolving an anionic drug and the cationic lipid represented by Chemical Formula 1 as defined above in an aqueous solvent respectively and mixing them, followed by freeze-drying; (b-1) dissolving the freeze-dried product obtained in step (a') in an organic solvent; (b-2) mixing the solution obtained in step (b-1) with an aqueous solvent; and (b-3) removing the organic solvent from the mixture obtained in step (b-2), wherein the amphiphilic block copolymer is dissolved in the organic solvent of step (b-1) or the aqueous solvent of step (b-2). (a') dissolving an anionic drug and the cationic lipid represented by Chemical Formula 1 as defined above in an aqueous solvent respectively and mixing them, followed by freeze-drying; (b-1) dissolving the freeze-dried product obtained in step (a') in an organic solvent; (b-2) mixing the solution obtained in step (b-1) with an aqueous solvent; and (b-3) removing the organic solvent from the mixture obtained in step (b-2), wherein the amphiphilic block copolymer is dissolved in the organic solvent of step (b-1) or the aqueous solvent of step (b-2).