According to the present invention, it is possible to improve the blood retention capability by a carrier for delivering a nucleic acid such as siRNA. A unit structure-type pharmaceutical composition of the present invention contains a block copolymer having a cationic polyamino acid segment and a hydrophilic polymer chain segment and a nucleic acid； the composition is electrically neutral by counterbalancing negative charge (s) of the nucleic acid with positive charge(s) of the cationic polyamino acid segment, and the nucleic acid is covered with the hydrophilic polymer chain segment.
