A nucleic acid-encapsulating polymer micelle complex is formed of a block copolymer containing an uncharged hydrophilic polymer chain block and a cationic polymer chain block； and two single-stranded DNA molecules having mutually complementary base sequences of 1000 or more bases in length, double-stranded DNA of 1000 or more base pairs in length in which at least a part of a double helix structure is dissociated and forms a single-stranded structure, or one single-stranded DNA molecule of 1000 or more bases in length.
