A unit structure consists of a single nucleic acid having a length of 10 to 30 bases, and a single block copolymer having a cationic polyamino acid segment and a hydrophilic polymer chain segment. In this unit structure: (i) a difference between a total of positive charges derived from cationic groups of the cationic polyamino acid segment and a total of negative charges derived from the nucleic acid in the unit structure falls within a range of ±10% of the total of the negative charges derived from the nucleic acid, (ii) the hydrophilic polymer chain segment comprises polyethylene glycol, (iii) the polyethylene glycol has a radius of inertia (Rg) equal to or longer than the length of the nucleic acid, and (iv) the cationic polyamino acid segment is bound to the nucleic acid via electrostatic bonds.
