A unit structure-type pharmaceutical composition includes a single nucleic acid, such as an antisense nucleic acid, electrostatically bound to a single block copolymer having a cationic polyamino acid segment and a hydrophilic polymer chain segment. The negative charges of the nucleic acid are counterbalanced, at least substantially, by the positive charges of the cationic polyamino acid segment such that the pharmaceutical composition is electrically neutral or nearly electrically neutral. Further, the nucleic acid is covered with the hydrophilic polymer chain segment. The block copolymer thereby improves the blood retention capability of the nucleic acids.
