It is an object of the present invention to provide: a polyion complex that sufficiently retains a photosensitizing substance in serum and is excellent in terms of structural stability; a nucleic acid polyplex as a constituent thereof; and a device and a kit for delivering a nucleic acid into a cell. The nucleic acid polyplex of the present invention comprises a cationic polymer represented by general formula (1) and a nucleic acid. The polyion complex of the present invention comprises the nucleic acid polyplex of the present invention and an anionic photosensitizing substance.