A unit-type polyion complex for use in delivering nucleic acid to a target site in a patient includes one or more molecules of a block copolymer having a poly(ethylene glycol) segment and a cationic poly(amino acid) segment and one or more molecules of a nucleic acid. A total quantity of positive charges derived from side chains of the cationic poly(amino acid) segment of the block copolymer in the unit-type polyion complex is not offset by a total quantity of negative charges derived from the nucleic acid. Furthermore, the nucleic acid has a strand length of 10-50 bases, the molecular weight of the poly(ethylene glycol) segment is 40×103 or more, and the block copolymer has a binding constant (Ka) for the nucleic acid of 3.0×105 or more.